redhat: parallel builds

redhat: parallel builds

Parallel builds can be an order of magnitude faster. Add the
_smp_mflags macro to all invocations of make in the rpm spec,
to make use of all available cores and SMT threads on the build
system. This should also help noticing new dependency issues
early. Note the macro can be overridden on the rpmbuild command
line.

Change-Id: Idddf8b867500d1ee73ff51de9d8a173bb4cc8c68
Reviewed-on: https://gerrit.openafs.org/13240
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>