cgal: build out of source. (#1918)
cgal: build out of source. (#1918)

* cgal: build out of source.

* Simplify cmake out-of-source.

LLNL/spack,skosukhin/spack,matthiasdiener/spack,LLNL/spack,krafczyk/spack,tmerrick1/spack,skosukhin/spack,skosukhin/spack,lgarren/spack,mfherbst/spack,matthiasdiener/spack,tmerrick1/spack,tmerrick1/spack,mfherbst/spack,iulian787/spack,matthiasdiener/spack,TheTimmy/spack,krafczyk/spack,EmreAtes/spack,skosukhin/spack,skosukhin/spack,TheTimmy/spack,iulian787/spack,tmerrick1/spack,TheTimmy/spack,krafczyk/spack,EmreAtes/spack,lgarren/spack,iulian787/spack,TheTimmy/spack,EmreAtes/spack,iulian787/spack,mfherbst/spack,lgarren/spack,TheTimmy/spack,LLNL/spack,EmreAtes/spack,LLNL/spack,krafczyk/spack,matthiasdiener/spack,LLNL/spack,lgarren/spack,iulian787/spack,mfherbst/spack,mfherbst/spack,tmerrick1/spack,krafczyk/spack,lgarren/spack,EmreAtes/spack,matthiasdiener/spack