Do not override LHCb compilation flags

Geant4 default configuration overrides compiler flags between the load of CMAKE_TOOLCHAIN_FILE and the test compilations CMake runs to validate the compiler.

This is not a major issue with gcc (apart from using options we do not expect), but it causes the configuration failure of clang because it overrides the mandatory option --gcc-toolchain=... that we set in the CMAKE_TOOLCHAIN_FILE.

This MR prevents Geant4 to overrides our settings.

Closes #3 (closed)