Isolate %_smp_build_ncpus and use it for %_smp_mflags.
Refactor _smp_build_ncpus and use it in %_smp_mflags. Note that now
having a single CPU, %_smp_mflags is expanded to '-j1'.
XXX: hand-edited to remove double quotes as per upstream commit
9b6fdc65ef0507fff04a69c88e085a7a26711839 which isn't applicable
directly due to other changes