This makes it convenient to build blender without referencing
pre-compiled libraries which don't always work on newer Linux systems.
Previously I had to rename ../lib while creating the CMakeCache.txt
to ensure my systems libraries would be used.
This change ensures LIBDIR is undefined when WITH_LIBS_PRECOMPILED is
disabled, so any accidental use warns with CMake's `--warn-unused-vars`
argument is given.
bc502f3b19