ENH: replace SOURCE_CGAL_VERSIONS_ONLY workaround (fixes #148)

- replace with an alternative workaround.

  Unset the _foamAddLib function prior to sourcing config.sh/CGAL:

  - LD_LIBRARY_PATH will not be adjusted.
  - cgal_version/boost_version variables will be retained.

Note: for ThirdParty builds, it is important that the boost and CGAL
paths are added into LD_LIBRARY_PATH even if before they are created.
This ensures that the OpenFOAM environment is functional after the
build.

The '*-none' and '*-system' specifications can still be used to avoid
setting them at all.