configure: properly check for mingw-w64 through installed headers.

mingw-w64 can also target 32-bit code.

Originally committed as revision 24156 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2271,7 +2271,7 @@ case $target_os in
         objformat="win32"
         enable dos_paths
         check_cflags -fno-common
-        if ! enabled x86_64; then
+        if ! check_cpp_condition _mingw.h "defined (__MINGW64_VERSION_MAJOR)"; then
             check_cpp_condition _mingw.h "(__MINGW32_MAJOR_VERSION > 3) || (__MINGW32_MAJOR_VERSION == 3 && __MINGW32_MINOR_VERSION >= 15)" ||
                 die "ERROR: MinGW runtime version must be >= 3.15."
             enabled_any avisynth vfwcap_indev &&