Cancel the force default libdir value.

On 64bits systems, the former version used to force the installation path
to be lib/ rather than x86_64-linux-gnu/lib
Also, when passing in the if condition, the lib install
path becomes in the .pc file
${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_PREFIX}/lib
which will cause some issues later when using those informations.
(This is the issue met by Karsten, for example).

With this commit, the install/lib folder has this structure, on 64 bits:
install/lib/python2.7
install/lib/x86_64-linux-gnu/plugin
install/lib/x86_64-linux-gnu/pkgconfig

So the debian multiarch system is restored for ubuntu 12.04.

Could you please verify that this does not destroy your system?

Regards