specifying BOOST_ROOT does nothing

[hariseldon78]

hello. the doc/build.md file says that you can specify BOOST_ROOT and other paths but that is not true. if i specify them with a -DBOOST_ROOT= on the cmake command it specifically says that the variable is unused. If i export it as an environment variable before the make command it just ignore it and download the whole package anyway. That is very time consuming, the only workaround is to replace the download url with a local path in the CMakeLists.txt.