Made build compatible with ROOT-6

Request for REVIEW AND TESTING, do NOT merge yet!

MGDO should now build and install also with ROOT-6 (and still with ROOT-5, of course).

Only minimal testing has been done, so far. Most changes only touch the build system though, and should not compromise correct function of BAT. However, a small code change BCHistogramFitter::CDF was necessary because ROOT-6 no longer provides `TH1::Integral(a, b, params , ...)`, this will have to be tested.