CONFIG: build boost, CGAL into lib64/ (issue #8)

- On 64-bit systems, the library locations for boost, CGAL are changing.

* Boost 1_62_0 and older build into 'lib/'.
* CGAL-4.9 builds into 'lib64/', older versions into 'lib/'.

Future-proof things by using lib$WM_COMPILER_LIB_ARCH for boost and
CGAL targets.