Cygwin: ffmpeg build tweaks

Cygwin: ffmpeg build tweaks
- bug: issues with universal input (including aac in mp4, wmv etc)
- fix: add -fno-common to GCC
- correctness: add --enable-memalign-hack as per ffmpeg docs
- these issues were possibly a regression introduced with the new build system
- a positive side effect to the build system is the availability of new makevar
  '<MODULE>.GCC.args.extra' which is a free-form way to add extra cflags to GCC functions.

[this changeset does not effect any other platforms]

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@2252 b64f7644-9d1e-0410-96f1-a4d463321fa5