Recognize FFMPEG_FORCE_NOCOLOR environment variable on Win32

Fixes issue 2461

Originally committed as revision 26186 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/log.c

@@ -55,7 +55,7 @@ static void colored_fputs(int level, const char *str){
 #if defined(_WIN32) && !defined(__MINGW32CE__)
         CONSOLE_SCREEN_BUFFER_INFO con_info;
         con = GetStdHandle(STD_ERROR_HANDLE);
-        use_color = (con != INVALID_HANDLE_VALUE) && !getenv("NO_COLOR");
+        use_color = (con != INVALID_HANDLE_VALUE) && !getenv("NO_COLOR") && !getenv("FFMPEG_FORCE_NOCOLOR");
         if (use_color) {
             GetConsoleScreenBufferInfo(con, &con_info);
             attr_orig  = con_info.wAttributes;