CMake improvements

[Flamefire]

@pdimov I created a PR for discussing proposed changes which is easier than the mailing list.

Basicall what I found:

• BUG: _Boost_INCLUDEDIR/LIBRARYDIR are not unset for interface targets
  • Moved down to where it was used and removed right after
• lot's of manual unset for "temporary" variables.
  • Used function which adds a scope. To set "persistent" variables one needs to explicitely use set(... PARENT_SCOPE). I believe this makes it much more clear what is used where and passed back
• Code duplication in generated files and lot's of (mostly) static CMake code embedded in the bjam files making maintenance harder
  • Factor out the search for variants into own function in own file.
  • Interface could probably be even further reduced to 1 parameter if the 2nd is always boost_<name>, but I wasn't entirely sure. Could you try to validate the behaviour especially for the (versioned) python stuff? I think there is just boost_python and a Boost_PYTHON_VERSION variable, isn't it?