[MLton] Finished (?) MLton.Child

[Stephen Weeks]

> mlton's 'system' call stopped working under both cygwin and linux too; what
> did you change? I see a "Process.wait (MLton.Process.spawnp (file = com,
> ..." in mlton/control/system.sml.
> 
> carrot:~$ mlton test.sml
> call to system failed with Function not implemented:
> gcc -c -I/usr/lib/mlton/include -O1 ....
> 
> Any ideas? It's stopping me from testing the current CVS.

Whoops.  I switched the test on forkIsEnabled in posix/process.sml.
I've checked in a fix.