This is essentially done by simply copying from gtk/fallback-c89.c https://bugzilla.gnome.org/show_bug.cgi?id=694339
The isnan() and isinf() are C99 macros not functions so use AC_CHECK_DECL instead of AC_CHECK_FUNCS for those.
Add a fallback-c89.c for the gdk/ subdirectory as there is code that uses functions that are introduced by C99. This currently adds fallback implementations for MSVC for isnan() and isinf() Dist this "new" source file as well
