visibility: Use a separate CFLAGS variable

We only want to control the default visibility for our five main
installable libraries: libglib, libgthread, libgmodule, libgobject,
libgio.  We should therefore only set -fvisibility=hidden when building
those.

Use a separate substitution variable for this purpose.

Using CFLAGS directly leads to some modules built in testcases not
exporting their symbols (and then the tests fail).  It also affects the
fam file monitoring module.

Colin had originally done it this way in his visibility patch series but
I failed to understand why so I didn't copy it.  Now I do.

Also: revert changes made to two testcases in an attempt to work around
this issue.

https://bugzilla.gnome.org/show_bug.cgi?id=691756

tests/module-test.c

@@ -27,16 +27,14 @@
 #undef G_DISABLE_ASSERT
 #undef G_LOG_DOMAIN
 
-#include "config.h"
-
 #include <gmodule.h>
 #include <string.h>
 
 gchar* global_state;
 
-_GLIB_EXTERN void g_clash_func (void);
+G_MODULE_EXPORT void g_clash_func (void);
 
-_GLIB_EXTERN void
+G_MODULE_EXPORT void
 g_clash_func (void)
 {
   global_state = "global clash";