Updated comments in Makefile.local for how to Diagnosing memory,
thread, and crash issues under RHEL and macOS.
We no longer support the Makefile variable ${ALLOW_CUSTOM} to be empty.
Normally ${ALLOW_CUSTOM} is:
ALLOW_CUSTOM= -DCUSTOM
Now, to disable custom disable custom even if -C is given, use:
ALLOW_CUSTOM="-UCUSTOM"
Added comments in Makefile.local for how to reduce dependency chains
under macOS. XXX - this doesn't yet work so don't uncomment - XXX.