gmodule: Add the visibility attribute to G_MODULE_EXPORT on gcc

For versions of GCC which support it (≥ 4), define G_MODULE_EXPORT as
__attribute__((visibility("default"))). This is normally a no-op, unless
compiling with -fvisibility=hidden, in which case it marks a symbol to
be publicly exported from the library, which is what G_MODULE_EXPORT is
for. Previously G_MODULE_EXPORT has only worked on Windows.

The compatibility check for whether the compiler supports
__attribute__((visibility)) is based on the __GNUC__ define, and is
similar to the check done in configure.ac for defining G_GNUC_INTERNAL.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=778287

gmodule/gmodule.h

@@ -35,7 +35,9 @@ G_BEGIN_DECLS
 #define	G_MODULE_IMPORT		extern
 #ifdef G_PLATFORM_WIN32
 #  define	G_MODULE_EXPORT		__declspec(dllexport)
-#else /* !G_PLATFORM_WIN32 */
+#elif __GNUC__ >= 4
+#  define	G_MODULE_EXPORT		__attribute__((visibility("default")))
+#else /* !G_PLATFORM_WIN32 && __GNUC__ < 4 */
 #  define	G_MODULE_EXPORT
 #endif /* !G_PLATFORM_WIN32 */