Special handling for some recognized env. vars.

CMake does not like that you specify the compiler with the environment
variables, instead preferring that you set them as cache variables.
This layer translate between the names of the the two.
Mimic Autotools way to specify non-standard Boost

If Boost is installed in say /usr/include/boost141 and
/usr/lib64/boost141, then there is no root you can specify to pick
them both up. However, whereas Autotools uses --with-boost and
--with-boost-libdir, FindBoost in CMake changes to using two different
variables: BOOST_INCLUDEDIR and BOOST_LIBRARYDIR. Using the header
directory for BOOST_ROOT will not work (in particularily not with
old CMake versions).
Merge pull request #351 from rolk/351_cxx

Alternate compiler cannot be set with environment variables