glib/gmessages.h: Unify log messages

glib/gmessages.h: Unify log messages

...Under various compilers when !G_DISABLE_CHECKS.  Previously, the
messages that are logged differ depending whether GLib was built with GCC
or not.  To simplify test cases, make all builds use a single output format
for g_return_if_fail(), g_return_val_if_fail(), g_return_if_reached(), and
g_return_val_if_reached(), by using the GCC-style format and replaceing
__PRETTY_FUNCTION__ with G_STRFUNC, so that it will work across various
compilers.

https://bugzilla.gnome.org/show_bug.cgi?id=711047