Fix Makefile bug: create the tmp dir before running GHC.
Fix Makefile bug: create the tmp dir before running GHC.

I didn't understand what was actually happening (I replaced 'tmp-opt'
with 'tmp/opt', but that didn't help), but manually creating the build
dir fixes the problem.  Someday I'll give up make for something more
sane ...

ntc2/haskell-call-trace