Add a simple "nproc" implementation

Our version of coreutils does not have "nproc" yet, so provide a simple
script-based implementation to allow "mgwport" to set MAKEOPTS to use an
appropriate number of make jobs.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Merge pull request #19 from sschuberth/nproc

Add a simple "nproc" implementation