Add checks on the _mingw.h and w32api.h versions.

Based on a patch by Ramiro.

Originally committed as revision 15793 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1405,6 +1405,11 @@ case $target_os in
         SHFLAGS='-shared -Wl,--output-def,$$(@:$(SLIBSUF)=.def) -Wl,--enable-runtime-pseudo-reloc -Wl,--enable-auto-image-base'
         objformat="win32"
         enable dos_paths
+        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_demuxer &&
+            { check_cpp_condition w32api.h "(__W32API_MAJOR_VERSION > 3) || (__W32API_MAJOR_VERSION == 3 && __W32API_MINOR_VERSION >= 12)" ||
+              die "ERROR: avisynth and vfwcap_demuxer require w32api version 3.12 or later."; }
         ;;
     cygwin*)
         target_os=cygwin