Allow controlling location of tmp_install

While running a couple of parallel make (install)check-worlds I was
surprised to see my system load at 5x what I'd think it'd be. Looking at
CPU % consumption it turns out this was because mds (the OS X indexing
service) and a few other things were going bonkers. That's because they
were all trying to process a huge amount of notification events.

I've now moved my normal install locations to somewhere I can exclude
from mds and what-not, but I don't see any way to do that with
tmp_install. Is there a trick here I'm missing, or should I change
Makefile.global.in to check for an environment variable?
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com
855-TREBLE2 (855-873-2532)