GNU make from git repository resolves an issue with a race condition around
make's pselect implementation.  Signals are lost, defunct child processes are
not reaped in a timely mannter.  The summary is that make -j4 can degrade to
-j1 functionally, etc.

Here's the commit that Paul Smith suggests actually fixes the issue:
http://git.savannah.gnu.org/cgit/make.git/commit/?id=b552b05251980f693c729e251f93f5225b400714

Example test data compiling Linux 4.17.2 with Tumbleweed's .config
* make_421 -j8 : 19-20 minutes
* make_421 -j4 : 32-60 minutes
* make_git -j4 : 18 minutes

This bug is a severe performance penalty on Tumbleweed, compared to Ubuntu
18.04