fixes #486 Revisit SOVERSION and VERSION

fixes #485 Honor BUILD_SHARED_LIBS
fixes #483 Don't expose private symbols in shared library
fixes #481 Export CMake target

This is a "large" commit involving changes that don't affect the
code directly, but which have an impact on how we package and build
our project.

The most significant of these changes is that we now build only
either a shared or a static library, depending on the setting of
the BUILD_SHARED_LIBS option.  We also suppress private symbols
from being exposed when the underlying toolchain lets us do so.

Minor updates to the way we version the ABI are used, and we now
have a nice exported CMake project.

To import this project in another, simply do find_package(nng)
and you can add target_link_libraries(nng::nng) to your targets.
CMake does the rest for you.

1 files changed, 6 insertions, 3 deletions

diff --git a/src/nng.h b/src/nng.h
index af119bdb..42ca0727 100644
--- a/src/nng.h
+++ b/src/nng.h
@@ -27,9 +27,8 @@ extern "C" {
 
 // NNG_DECL is used on declarations to deal with scope.
 // For building Windows DLLs, it should be the appropriate __declspec().
-// (We recommend *not* building this library as a DLL, but instead linking
-// it statically for your project to minimize concerns about link
-// dependencies later.)
+// For shared libraries with platforms that support hidden visibility,
+// it should evaluate to __attribute__((visibility("default"))).
 #ifndef NNG_DECL
 #if defined(_WIN32) && !defined(NNG_STATIC_LIB)
 #if defined(NNG_SHARED_LIB)
@@ -38,7 +37,11 @@ extern "C" {
 #define NNG_DECL __declspec(dllimport)
 #endif // NNG_SHARED_LIB
 #else
+#if defined(NNG_SHARED_LIB) && defined(NNG_HIDDEN_VISIBILITY)
+#define NNG_DECL __attribute__((visibility("default")))
+#else
 #define NNG_DECL extern
+#endif
 #endif // _WIN32 && !NNG_STATIC_LIB
 #endif // NNG_DECL