Add information about a workaround for MinGW gcc versions.

based on a patch by Art Clarke, aclarke vlideshow com

Originally committed as revision 15670 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/general.texi

@@ -458,10 +458,13 @@ are listed below:
 @item mingw-runtime 3.15
 @end itemize
 
+You will also need to pass @code{-fno-common} to the compiler to work around
+a GCC bug (see @url{http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37216}).
+
 Within the MSYS shell, configure and make with:
 
 @example
-./configure --enable-memalign-hack
+./configure --enable-memalign-hack --extra-cflags=-fno-common
 make
 make install
 @end example