If a build expressions has set "enableParallelBuilding = true", then the
generic builder may utilize more than one CPU core to build that particular
expression. This feature works out of the box for GNU Make. Expressions that
use other build drivers like Boost.Jam or SCons have to specify appropriate
flags such as "-j${NIX_BUILD_CORES}" themselves.
svn path=/nixpkgs/trunk/; revision=23042
