From the GNU make 3.82 release announcement:
* The 'define' make directive now allows a variable assignment operator
after the variable name, to allow for simple, conditional, or appending
multi-line variable assignment.
macOS ships with 3.81. This caused the multiprocess config options
to be ignored.
Fixes#32068
Github-Pull: #32070
Rebased-From: 9157d9e449870851ef455e077249ac46fc2df24c
Co-authored-by: Ryan Ofsky <ryan@ofsky.org>