Added support for parallel builds

With this patch, one can define the degree of build parallelism via a 'jobs'
rpm variable. A comfortable way to use this is having a ~/.rpmmacros file with
a line like:

   %jobs 2

Alternatively, one could use a '--define "jobs 2"' command line flag for
rpmbuild. On a Core 2 Duo using 2 jobs brings down the time for a full build
including extralibs from 36m to 27m. If 'jobs' is not defined, a normal
sequential build is done, following the usual conventions on e.g. openSUSE.