Fix the plugins install path, and search path in "*.pc" files

Fix the plugins install path, and search path in "*.pc" files

Plugins were not always installed in a directory matching
the architecture ("/usr/lib" instead of "/usr/lib64" e.g.),
which caused build problems.

Depending on the architecture, libraries may be installed
in another directory than "/usr/lib" ("/usr/lib64" e.g.)
Reflect that in the .pc files.

Change-Id: I286907a4b3d59e3416ca96e8903665f155c07415
Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>