being used.

We now track whether a module used any TH splices in the ModIface (and
at compile time in the TcGblEnv and ModGuts).  If a module used TH
splices last time it was compiled, then we ignore the results of the
normal recompilation check and recompile anyway, *unless* the module
is "stable" - that is, none of its dependencies (direct or indirect)
have changed.  The stability test is pretty important - otherwise ghc
--make would always recompile TH modules even if nothing at all had
changed, but it does require some extra plumbing to get this
information from GhcMake into HscMain.

test in driver/recomp009