Reverted CMake changes.

If the Boost libraries are installed in a non-standard location, e.g.
one the CMake doesn't know about, just run the build with:

cmake --DBOOST_ROOT=<path to the boost libraries>

For example on my system, assuming I'm in a build sub-directory, I'd use:

cmake -DBOOST_ROOT=/opt/modules/boost/1.56.0 ../