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.

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STYLE: check for boost/version.hpp, CGAL/version.h instead their directories