premake: require getconf at build time
premake: require getconf at build time

This avoids make being called with "-j" alone.

moochris/haikuports,moochris/haikuports,nielx/haikuports,nielx/haikuports,haikuports/haikuports,nielx/haikuports,haikuports/haikuports,Begasus/haikuports,haikuports/haikuports,moochris/haikuports,haikuports/haikuports,Begasus/haikuports,Begasus/haikuports,haikuports/haikuports,Begasus/haikuports,Begasus/haikuports