This has been discussed in the past, see http://slicer-devel-archive.65872.n3.nabble.com/Reducing-Slicer-build-time-on-Windows-using-CMAKE-CXX-MP-FLAG-td4035926.html
@adamrankin How much does it reduce the build time for you?
As I remember, there were some potential issues in parallel builds - but maybe it was about parallel execution of test. @jcfr is there any risk or disadvantage of enabling parallel builds?