configure: Move MinGW CPPFLAGS setting to libc section, where it belongs

configure

@@ -2915,7 +2915,6 @@ case $target_os in
         dlltool="${cross_prefix}dlltool"
         ranlib=:
         enable dos_paths
-        add_cppflags -U__STRICT_ANSI__
         ;;
     win32|win64)
         if enabled shared; then
@@ -3057,6 +3056,7 @@ elif check_header _mingw.h; then
         "defined (__MINGW64_VERSION_MAJOR) || (__MINGW32_MAJOR_VERSION > 3) || \
             (__MINGW32_MAJOR_VERSION == 3 && __MINGW32_MINOR_VERSION >= 15)" ||
         die "ERROR: MinGW runtime version must be >= 3.15."
+    add_cppflags -U__STRICT_ANSI__
 elif check_func_headers stdlib.h _get_doserrno; then
     libc_type=msvcrt
     add_compat strtod.o strtod=avpriv_strtod