constants file

[Stephen Weeks]

I just checked in a bunch of cross-compiler changes.  One issue that
has come up is that I have created a constants file that contains the
values of all of the target constants (e.g. ITIMER_PROF) that the
basis library needs.  The question is, for the rpm, when should I
create this file.  I see three possibilities.

1. At each invocation of mlton
2. At rpm install time
3. At rpm build time

(1) is what we used to do with the old approach to constants.  (3) is
what I'm doing now.  But (2) seems right.  Any opinions?