As others have said,
before you run sbopkg or an SlackBuild, but BEWARE! Some of them (very few) *will* go horribly wrong when you do this . The usual way this shows up is a mysterious "file not found" error at the linking stage. To get round it, just
for the problem SlackBuilds.
 including (but not limited to) gsm, gsoap, john.
Edit: I think cmake uses MAKEFLAGS ok :-)