Respect the CMAKE_INSTALL_PREFIX for the library installs since it's now
referenced by the "PCM_LIB_DIR".  In most cases this will resolve to /usr,
as such drop "/usr" from the LIB_INSTALL_DIR.  Perhaps I should just set
LIB_INSTALL_DIR to $(get_libdir) and move the beignet ocl bits into place
afterwards as is done for Clover?