Maybe no one cares anymore, but I've discovered that if I run make with -j the installation sometimes fails with this error:

install: mkdir /Users/build/.../dist/python/x86_64-darwin/lib: File exists

I believe it's because the targets altbininstall and libainstall as well as $(DESTSHARED) ($(BINLIBDEST)/lib-dynload) all contain a for loop which tries to create $(LIBDIR).  The makefile uses the install -d command to create directories and this command will fail if the directory already exists.

I haven't investigated the full dependency chain but at least two of the above targets don't have a relationship that forces make to avoid running them both at the same time.

Maybe a better solution would be to create a separate target like make-directories or something that creates all the directories and have the other targets depend on that one target.  Or something.

As it is, my MacOS builds fail about 1 in 5 times or similar.

Interestingly my Linux builds never fail.  Not sure if install works differently on Linux, or the timing is just different there.