CMake: Set LCB_LIBDIR in config.h

This makes the compilation line less noisy

Change-Id: I1c491b55077edb974ade362f15a664391f4fa8a5
Reviewed-on: http://review.couchbase.org/42773
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>