LibXml2 Handling Fixes, 1.0 branch (2017.11.07.)

Turns out that the CMake build was using libxml2 incorrectly all along. Which now showed up in ATLASG-1352.

This update teaches AtlasLCG how to handle libxml2 dependencies in the ATLAS code correctly, setting it up properly for both the build of the software, and then later for its runtime as well.

While doing this, I also backported some of the changes of the main LCG configuration file into the 1.0 branch, as this sort of setting actually works correctly, contrary to what we had in the code before.