ENH: improved handling of gmp/mpfr library settings (issue #674)

- export library path for gmp/mpfr from CGAL config files.
  This is required when non-system gmp/mpfr libraries are being
  used, but not using a ThirdParty compiler installation.

- automatically handle lib/ vs lib64/ (eg, for central installations)
  for packages such as boost, CGAL, etc. While the ThirdParty
  compilation of these will normally land in lib64/, this may not be
  the case when they are supplied by another means.

- reworked the handling of foamEtcFile and foamCleanPath for less
  clutter in the configuration files.
  Added the bin/tools/lib-dir script to handle logic that is
  too complex to easily manage in csh.