Support to use GNUInstallDirs

CMake provides the module GNUInstallDirs which is taking care of defining the proper install paths depending on the platform, so we don't need to hardcode install paths any more.

This will change default install paths in Debian and Ubuntu distribution (except Precise) to be: lib/$arch/ instead of just lib. The system is prepare to work with this multiarch paths  and the pkgconfig file has been changed according.

Let me know if you see any potential problem with this.