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.c

@@ -167,9 +167,15 @@
 /**
  * G_MODULE_EXPORT:
  *
- * Used to declare functions exported by modules. This is a no-op on Linux
+ * Used to declare functions exported by libraries or modules.
- * and Unices, but when compiling for Windows, it marks a symbol to be
+ *
- * exported from the library or executable being built.
+ * When compiling for Windows, it marks the symbol as `dllexport`.
+ *
+ * When compiling for Linux and Unices, it marks the symbol as having `default`
+ * visibility. This is no-op unless the code is being compiled with a
+ * non-default
+ * [visibility flag](https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fvisibility-1260)
+ * such as `hidden`.
  */
 
 /**

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 */