configure: add __USE_MINGW_ANSI_STDIO on MinGW

This makes MinGW redirect certain stdio functions (such as the sprintf
family) from the MSVCRT libc to a standard compliant MinGW
implementation.

This fixes a crash in talloc.c when compiling mplayer with MinGW-w64.
The problem is most likely with talloc_vasprintf(), which calls
vsnprintf with a small buffer and checks its return value to find out
how much space the formatted string requires. Without this commit,
vsnprintf would always return -1, and then the code calls abort().
(lachs0r figured out this one.)

configure

@@ -1397,6 +1397,7 @@ fi
 if mingw32 ; then
   _getch=getch2-win.c
   need_shmem=no
+  extra_cflags="$extra_cflags -D__USE_MINGW_ANSI_STDIO=1"
 fi
 
 if amigaos ; then