Pick up stuff from GLib to hide symbols with Sun C.

Note why we don't use G_GNUC_INTERNAL, but duplicate what GLib does, and
don't use G_HAVE_GNUC_VISIBILITY to determine whether we can use
__attribute__ ((visibility (...))).

Change-Id: I0b8d40f40d04e821352522320626173806787214
Reviewed-on: https://code.wireshark.org/review/2753
Reviewed-by: Guy Harris <guy@alum.mit.edu>