Don't consider files in the Cellar as Racks

Otherwiese a `Cellar/.DS_Store` may lead to an error about
`.ds_store.rb` not found during `brew upgrade`.
avidemux: 2.6.4, --HEAD and builds with clang

A major rework of this formula.
Removed patch and some inreplace and fixes we needed earlier.
Need to deparallelize the core build, too.

Unfortunately, still doesn't build with SDL.
In the Qt-GUI it reports that the build options don't include
gettext, freetype etc. but in fact we added all of them and during
the build Avidemux seems to pick them up correctly. Therefore, the
GUI reporting code is perhaps broken.