On 64-bit systems, the system installations of boost, cgal are under
lib64/. The behaviour for a ThirdParty build is mostly lib/ but this
can also be 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 build rules, and forcing these as build targets in the ThirdParty
makeCGAL as well.
--
STYLE: check for boost/version.hpp, CGAL/version.h instead their directories
