Always assume that we use a gnu/c99 printf implementation

On Windows we use gnulib and elsewhere we use glibc or similar.

Also change G_GNUC_PRINTF to use gnu_printf instead of __format__ if
possible because __format__ evaluates to ms_printf under MinGW,
but we use gnulib there and not the system printf.
gnu_printf is only available with GCC>=4.4 and not with clang.

https://bugzilla.gnome.org/show_bug.cgi?id=795569