Cancel the force default libdir value.

On 64bits systems, this forces 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.
Merge pull request #22 from francois-keith/master

Cancel the force default libdir value.