if BOOST_SOURCE_DIR is defined in ENV variable,
and is not overruled by -DBOOST_SOURCE_DIR at cmake invocation
then we use it.
If BOOST_SOURCE_DIR is not defined in ENV variable,
and not defined by -DBOOST_SOURCE_DIR,
then we look for the system compiled boost.
-> allows to set up BOOST_SOURCE_DIR in your bashrc
on machine where boost compilation is needed, once for all
new policy : for non standard conforming compilers,
we can skip the tests using new features using recent c++ features.
I.e. compilers below gcc 4.8.1, clang 3.3.
