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).