Pass $CC as AS, unless $AS is provided. Add gas-preprocessor

Add gas-preprocessor.pl as a git submodule, and put it in the
path so that it is available if libav wants it.

Switch back to providing $CC as $AS by default, but
respect an external $AS setting so that it can be
overridden in cerbero

https://bugzilla.gnome.org/show_bug.cgi?id=694416