don't use "deprecated" ANSI/POSIX calls on Windows GDK

These functions are not available when linking, so we should avoid using them. The preferred way is to use the variants with an underscore.

Also mingw-w64 understands _CRT_NONSTDC_NO_DEPRECATE but not _CRT_NONSTDC_NO_WARNINGS.

This allows building the core of VLC when compiling with the GDK.