Find libClang properly
This MR allows to find libClang properly. It also updates the LCG default version to LCG_99.
- Provide FindLLVM.cmake, FindLibClang.cmake and FindClang.cmake.
- Require minimum major version 9 for found libClang.
- Properly use environment variables
LD_LIBRARY_PATH
andCPLUS_INCLUDE_PATH
inGenerateConfiguration.cmake
.