Use nproc instead of the home-brewed solution

Signed-off-by: Thomas Braun <thomas.braun@byte-physics.de>
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Merge pull request #22 from sschuberth/use-nproc

mingw32-git: Use the "nproc" tool to determine the number of processors