build: Fix find_library for contrib avutil on GNU/Linux.

* src/libclient/CMakeLists.txt: Set the right 'hint' path for the call
to find_library for ffmpeg avutil from daemon's contrib, to match the
directory name used by daemon's 'contrib/bootstrap' script: the name
of the directory is the value returned by the C++ compiler (either GNU
or Clang) when invoked with the '-dumpmachine' option.

Change-Id: I03de150aa941a1e32471e5b4cbe507e5d09787f5