[VECGEOM-281] Quote CMake variables to avoid problems with policy CMP0054
Unquoted variables were showing the follwing error on Jenkins:
CMake Error at VecGeomConfig.cmake:28 (if):
if given arguments:
"STREQUAL" "GNU"
Unknown arguments specified
Call Stack (most recent call first):
cmake/modules/FindVecGeom.cmake:23 (find_package)
CMakeLists.txt:170 (find_package)
Closes VECGEOM-281.
Note: This fixed the quoting problem, but we still need to fix the VecGeomConfig.cmake file not to mess with CMAKE_CXX_FLAGS directly, as that is a bug.
Note 2: JIRA integration is still broken for VecGeom, so the "Closes VECGEOM-281" above is not linked, and neither will it close the issue at hand. Please fix that.