AthAnalysisExternals Updates, 1.0 branch (2019.07.05.)

These two updates are necessary to make AthAnalysisExternals build successfully for the x86_64-centos7-gcc8-opt platform with CMake 3.14.

• I had to turn off the build of CheckerGccPlugins for such new versions of GCC. I assume this is okay with you @ssnyder. It didn't seem to make sense to me to invest time in making that code work with GCC 7/8 in this branch.
• I found that the CLHEP build configuration was actually quite buggy so far, which only caused a build error with the latest CMake version. I now tried to clean up that package properly.