CentOS 7 Fixes, 1.0 branch (2019.07.12.)

These updates are necessary to fix the build issues seen in the first x86_64-centos7-gcc8-opt nightlies of AnalysisBase and AnalysisTop. Which I described in ATLINFR-3120.

I have to admit I still don't understand why I didn't see issues earlier on in my local builds of AnalysisBaseExternals, as the python-devel RPM package is installed on my private machine as well. Still, for some reason on the nightly node FindPythonInterp.cmake found Python under /bin, while on my own machine it found it under /usr/bin. 😕 The update in ProjectOptions.cmake should now make the code ignore both.

At the same time, since the real trigger for the ROOT build failure in the nightly was that because of the locally installed Python version /lib64 was added to LD_LIBRARY_PATH in the project's env_setup.sh file, I also made some changes in AtlasLCG to stop this from happening ever again.

With these two changes I was able to build AnalysisBaseExternals and AnalysisBase successfully on aibuild16-006, so this should be enough to get the nightly working...