fixes #735 Configuring ZeroTier for a unified build is too hard

This changes the code to make use of a different project we have
created (libzerotiercore) that is "CMake clean". This should make
using and configuring this code *much* better.  It may also have the
benefit of making this configuration work better for Windows systems.

4 files changed, 49 insertions, 68 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3042be38..52421a0b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -236,6 +236,7 @@ mark_as_advanced(NNG_TRANSPORT_WSS)
 
 option (NNG_TRANSPORT_ZEROTIER "Enable ZeroTier transport (requires libzerotiercore)." OFF)
 if (NNG_TRANSPORT_ZEROTIER)
+    find_package(zerotiercore REQUIRED)
     add_definitions (-DNNG_TRANSPORT_ZEROTIER)
 endif ()
 mark_as_advanced(NNG_TRANSPORT_ZEROTIER)
diff --git a/docs/BUILD_ZEROTIER.adoc b/docs/BUILD_ZEROTIER.adoc
index a61d065c..80ee0722 100644
--- a/docs/BUILD_ZEROTIER.adoc
+++ b/docs/BUILD_ZEROTIER.adoc
@@ -5,48 +5,41 @@ endif::[]
 = Building for ZeroTier Support
 
 If you want to include support for the experimental ZeroTier
-transport (zt:// URLs), you should follow these directions.
+transport (`zt://` URLs), you should follow these directions.
 
-You will need a checked out copy of the ZeroTierOne repository.
-The repo is located at github.com/ZeroTier/ZeroTierOne.
-These instructions have been tested with version 1.2.10 (tag)
-of that repository.
+Staysail has created a package that builds a clean CMake project
+that you can import.
+Download this from https://github.com/staysail/libzerotiercore[GitHub].
 
-IMPORTANT: ZeroTier is licensed under different terms than NNG.
-You are responsible for reading those license terms, and ensuring
-that your use conforms to them.
 
-IMPORTANT: The ZeroTier transport, and these build instructions, are
-experimental.  We expect both to change over time as ZeroTier
-and the NNG zt:// transport both mature.  As always, _caveat emptor_.
+IMPORTANT: ZeroTierOne is licensed under different terms than NNG.
+You are responsible for reading those license terms, and ensuring that
+your use conforms to them.
 
-== Building libzerotiercore.a
+IMPORTANT: The ZeroTier transport, and these build instructions,
+are experimental.
+We expect both to change over time as ZeroTier and the NNG `zt://` transport
+both mature.
+As always, _caveat emptor_.
 
-The ZeroTierOne repo is not set up for building this library by default,
-so you'll need to explicitly build it.
+== Building libzerotiercore
 
-=== Using "make"
-If you have a "make" based system such as Linux or macOS, you can
-just do "make core" at the top-level.  The end result will be a
-libzerotiercore.a in that top level.
-
-=== Using "cmake"
-
-If not, you can use the CMakeLists file at the top of the ZeroTierOne tree.
-That CMakeLists file only generates one output, the libzerotiercore.a
-library, which will normally be a static delivery.
-
-Using your usual cmake tool chain, build the library.
-The resulting library (usually `libzerotier.a`) will need to be copied
-into the top-level of the ZeroTierOne tree, as that is where the NNG
-build expects to find it.
+Using "CMake", you can build libzerotiercore
+using any normal CMake generator.
+You should install it as well.
+Normally it will install into `/usr/local` on
+Linux and UNIX systems.
 
 == Configuring NNG with ZT
 
-You will need to enable ZeroTier within NNG using the CMake option
-`NNG_TRANSPORT_ZEROTIER=ON` and set the CMake option
-`NNG_TRANSPORT_ZEROTIER_SOURCE` to the directory path of the ZeroTierOne
-tree you have checked out.
+You will need to enable ZeroTier within NNG using the CMake option `NNG_TRANSPORT_ZEROTIER=ON`.
+If you have installed the libzerotiercore in
+one of the default locations, then that should be all
+that is needed.
+
+If you have specified a different location, you will
+need to inform CMake of this by setting the CMake
+variable `-Dzerotier_DIR=<where you installed it>`
 
 == Example
 
@@ -56,23 +49,24 @@ that we have checked out github source trees into `$HOME/work`.
 [source, sh]
 ----
 $ export NNGDIR=$HOME/work/nng
-$ export ZTDIR=$HOME/work/ZeroTierOne
+$ export ZTDIR=$HOME/work/libzerotiercore
+$ git clone https://github.com/staysail/libzerotiercore $ZTDIR
 $ cd $ZTDIR
 $ mkdir build
 $ cd build
+$ cmake ..
 
    ... (lots of lines of output from cmake...)
 
-$ cmake ..
 $ make
 
    ... (lots of lines of output from make...)
 
-$ cp libzerotiercore.a $ZTDIR
+$ make install
 $ cd $NNGDIR
 $ mkdir build
 $ cd build
-$ cmake -DNNG_TRANSPORT_ZEROTIER=ON -DNNG_TRANSPORT_ZEROTIER_SOURCE=${ZTDIR} ..
+$ cmake -DNNG_TRANSPORT_ZEROTIER=ON ..
 
    ... (lots of lines of output from cmake...)
 
diff --git a/src/transport/zerotier/CMakeLists.txt b/src/transport/zerotier/CMakeLists.txt
index 8ca78f0f..d8dbb566 100644
--- a/src/transport/zerotier/CMakeLists.txt
+++ b/src/transport/zerotier/CMakeLists.txt
@@ -10,43 +10,29 @@
 
 # ZeroTier protocol
 
-set (NNG_TRANSPORT_ZEROTIER_SOURCE "" CACHE PATH "Location of ZeroTier source tree.")
-mark_as_advanced(NNG_TRANSPORT_ZEROTIER_SOURCE)
-
 if (NNG_TRANSPORT_ZEROTIER)
 
-    # We use the libzerotiercore.a library, which is unfortunately a C++ object
-    # even though it exposes only public C symbols.  It would be extremely
-    # helpful if libzerotiercore didn't make us carry the whole C++ runtime
-    # behind us.  The user must specify the location of the ZeroTier source
-    # tree (dev branch for now, and already compiled please) by setting the
-    # NNG_ZEROTIER_SOURCE macro.
-    # NB: This needs to be the zerotierone tree, not the libzt library.
-    # This is because we don't access the API, but instead use the low
-    # level zerotiercore functionality directly.
+    # The zerotiercore project will have been found at the top level
+    # of the package.  This is necessary because import libraries cannot
+    # percolate back to the parent tree.
+
     # NB: As we wind up linking libzerotiercore.a into the application,
     # this means that your application will *also* need to either be licensed
     # under the GPLv3, or you will need to have a commercial license from
     # ZeroTier permitting its use elsewhere.
-    
-    enable_language(CXX)
-    find_library(NNG_LIBZTCORE zerotiercore PATHS ${NNG_TRANSPORT_ZEROTIER_SOURCE})
-    if (NNG_LIBZTCORE)
-        set(CMAKE_REQUIRED_INCLUDES ${NNG_TRANSPORT_ZEROTIER_SOURCE}/include)
-        message(STATUS "C++ ${CMAKE_CXX_IMPLICIT_LINK_LIBRARIES}")
-        set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} ${NNG_LIBZTCORE} ${CMAKE_CXX_IMPLICIT_LINK_LIBRARIES})
-        set(NNG_REQUIRED_LIBRARIES ${NNG_REQUIRED_LIBRARIES} ${NNG_LIBZTCORE} ${CMAKE_CXX_IMPLICIT_LINK_LIBRARIES} PARENT_SCOPE)
-        set(NNG_REQUIRED_INCLUDES ${NNG_REQUIRED_INCLUDES} ${NNG_TRANSPORT_ZEROTIER_SOURCE}/include PARENT_SCOPE)
-        nng_check_sym(ZT_Node_join ZeroTierOne.h HAVE_ZTCORE)
-    endif()
-    if (NOT HAVE_ZTCORE)
-        message (FATAL_ERROR "Cannot find ZeroTier components")
-    endif()
-    message(STATUS "Found ZeroTier at ${NNG_LIBZTCORE}")
+
+    message(WARNING "
+        ************************************************************
+        Linking against zerotiercore changes license terms (GPLv3).
+        Consult a lawyer and the license files for details.
+        ************************************************************")
+
+    set(NNG_REQUIRED_LIBRARIES ${NNG_REQUIRED_LIBRARIES} zerotiercore::zerotiercore)
+    set(NNG_REQUIRED_LIBRARIES ${NNG_REQUIRED_LIBRARIES} PARENT_SCOPE)
 
     set(ZT_SOURCES transport/zerotier/zerotier.c transport/zerotier/zerotier.h)
     set(ZT_HEADERS transport/zerotier/zerotier.h)
-endif()
 
-set(NNG_SOURCES ${NNG_SOURCES} ${ZT_SOURCES} PARENT_SCOPE)
-set(NNG_HEADERS ${NNG_HEADERS} ${ZT_HEADERS} PARENT_SCOPE)
+    set(NNG_SOURCES ${NNG_SOURCES} ${ZT_SOURCES} PARENT_SCOPE)
+    set(NNG_HEADERS ${NNG_HEADERS} ${ZT_HEADERS} PARENT_SCOPE)
+endif()
diff --git a/src/transport/zerotier/zerotier.c b/src/transport/zerotier/zerotier.c
index 1fc128b7..0cd83ee7 100644
--- a/src/transport/zerotier/zerotier.c
+++ b/src/transport/zerotier/zerotier.c
@@ -16,7 +16,7 @@
 #include "core/nng_impl.h"
 #include "zerotier.h"
 
-#include <ZeroTierOne.h>
+#include <zerotiercore/ZeroTierOne.h>
 
 // ZeroTier Transport.  This sits on the ZeroTier L2 network, which itself
 // is implemented on top of UDP.  This requires the 3rd party