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