mingw32: properly check if vfw capture is supported by the system headers

mingw32: properly check if vfw capture is supported by the system headers

Remove check for an specific w32api version, checking instead if vfw.h
supports vfw capture. The defines in w32api 3.12 were wrong, so this must be
accounted for in the check.

Originally committed as revision 24203 to svn://svn.ffmpeg.org/ffmpeg/trunk