HEAD build failures

[Ross Paterson]

I'm getting a mysterious failure in Text.Regex (in regex-compat).
It can be kludged by adding ::ExecOption to one of the defaultExecOpt's
there (after importing ExecOption), but on my reading of the FDs this
shouldn't be necessary.  Perhaps it's connected to all the orphan
instances in these packages.

Also, it seems that compiler/Makefile is missing dependencies for regex-*
for stages 2 and 3.