Simplify C++ compiler flag setup and use

The end point of this MR will be to export compiler flags required by clients of libvecgeom to CMake/pkg-config files so that these are applied automatically. The canonical case is the arch flag as this determines the (maximum) ISA compiled into the library. As submitted, the changes are:

• Rationalize setting of Clang/Intel/GNU flags so these appear in the same order and are as common as can be
  • Add error limit reporting for Intel/Clang
  • Intel can support -ggdb
• The explicit setting of -fPIC is removed, replaced by CMAKE_POSITION_INDEPENDENT_CODE
• Provide C++17 support for Intel compilers using usage requirements
• De-duplicate flags that appear in both VECGEOM_CXX_FLAGS and VECGEOM_COMPILATION_FLAGS, only setting them in the latter

This last step is the foundation for the export of flags - it contains the flags that must be used by clients of libvecgeom when they compile/link against it. That export isn't done yet as I would like to get a cycle of testing in just to confirm the flag changes don't break things.

I'd also like a readout/confirmation that the current VECGEOM_CXX_FLAGS and VECGEOM_COMPILATION_FLAGS (ignoring the ISA flags) are o.k., whether they are still required, and why we need them. For example -diag-disable 3438 on Intel. I'll fixup any issues here and add comments on why we're using the flag in question.