clean up CMakeLists.txt files for inclusion in other projects
- only set compilation settings when SOAContainer is built as standalone project. Otherwise do not override external settings. DRAWBACK: I am not aware of a clean and reliable way to detect the C++ standard set by the external project, so atm no warning.
- Provide option SOAContainer_header_destination to determine installation path for header files.
- Link guitest against TBB (potential not-forwarded dependency of ROOT)
- Disable -Wsuggest-override for googletest (fails build)
- Add SYSTEM keyword to include_directories calls where considered appropriate (including not-under-our-control headers)