* generate pkg-config files top level to make sure all
exports are available (eg. client/server)
* .pc libs section is now generated from export libraries
* update .pc file to use .private fields (prevent overlinking)
On Gentoo Linux, CMAKE_INSTALL_LIBDIR is passed as an absolute path,
which caused plugins to be installed in /usr/usr/lib/freerdp.
The new logic better emulates the behavior of the real GNUInstallDirs module.