Fix string replace in environment in *Config.cmake for projects with the same prefix
In particular, when replacing PROJECT_SOURCE_DIR of Gaussino, GaussinoExtLibs is affected as well resulting in variables set in GaussinoExtLibs not being recognized in downstream projects. This should fix the problems with Geant4 variables:
- lhcb/Geant4#9 (closed)
- lhcb/Geant4#8 (closed)
- lhcb/Gauss#78
- lhcb/Gauss!885 (merged) will no longer be needed
To be ported to lhcb/LHCbConfigUtils.cmake