x264enc: Add configure parameter to specify additional x264 libraries with e.g. diffe...

x264enc: Add configure parameter to specify additional x264 libraries with e.g. different depth configuration

x264 has to be compiled specifically for a target bit depth.
Distributions currently ship various libraries in their packages, with
different bit depths.

This change now allows to provide them all at configure time and have
the x264enc element dynamically switch between them based on the bit
depth of the input format.

https://bugzilla.gnome.org/show_bug.cgi?id=763297