Accurate G_HAVE_GNUC_VISIBILITY is needed to correctly
define G_GNUC_INTERNAL later on. Autotools did that,
meson currently doesn't and opts to just set
G_HAVE_GNUC_VISIBILITY to 1 for all compilers except MSVC.
This leads to MinGW GCC having G_HAVE_GNUC_VISIBILITY=1,
which results in G_GNUC_INTERNAL being defined to
__attribute__((visibility("hidden"))), which is not supported.
Because cc.compiles() does not support override_options or
anything like that, we just feed it '-Werror' as-is, since
MSVC is known as not supporting visibility attributes anyway.
https://bugzilla.gnome.org/show_bug.cgi?id=794636
Properly define GLIB/GOBJECT_STATIC_COMPILATION when static build is enabled.
Use library() instead of shared_library() to allow selecting static builds.
https://bugzilla.gnome.org/show_bug.cgi?id=784995
They are not supported by Visual Studio, so only define them in
glibconfig.h.in when not on Visual Studio. Fixes builds of GTK+-2.x
against Meson/MSVC builds of GLib.
https://bugzilla.gnome.org/show_bug.cgi?id=783270
