Jobserver fifo
Recent GNU make versions support opening a fifo path instead of using passed filedescriptor numbers. This simplifies using the jobserver functionality in build systems like openembedded.
These two patches are cherry-picked from https://github.com/ninja-build/ninja/pull/2263 while skipping the patches adding joberserver master support.
A few simple conflicts were resolved in the test file while cherry-picking.
@bradking Can you take a look on this, or ping someone who can?
@hundeboll thanks. In general we don't accept pull requests directly to this fork. It exists only to stage upstream PRs that haven't been merged there yet. This PR is toward that goal, but I'd like to avoid cherry-picking if possible. When I get a chance I'll look through the current state of the relevant upstream PRs to see if I can rebuild our fork.