Use a portable method to link against internal FFMPEG

In the last iteration, we kept the original method to link the shared
plugin and edited the .a and .la files so satisfy what cerbero needed.
Unfortunately, that required adding .a file into the archive which is
not allowed with iOS ar command for universal builds.

This patch uses standard method to link a static library. One of the
benefit is that it removes some libtool warning about portability.
For the static case, we implement an install hook that installs
FFMPEG internal .a files in the plugin directory (so it does not get
confused with a possible system FFMPEG. This makes the static plugin
usable without depending on cerbero recipe.