fix faad2 version check

fix faad2 version check

On fedora 22, the output of cpp inserts extra debug comments, which
makes our regexp for the faad2 version check fail. This in turn causes
it to compile with the wrong arguments passed which then causes stack
corruption and crashes.

Fix this by only checking for the version (which should be by itself on
a single line). This is potentially less safe, it might be possible that
a similar string would appear in a later version in the header file.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=748571