add option to limit job execution dependant on memory consumption (-m)

Setting a value in range [0,100] for -m limits starting of new jobs. once
the limit has been exceeded at most one single job is run at a time.

This aims at C++ make projects that run into memory limitations when
building expensive (in regards to memory) compilation units, e.g. caused
by massive template instantiations.

The implementation currently covers support for Apple, Linux and Windows.

Signed-off-by: Matthias Maennich <matthias@maennich.net>