Do away with that 'ineffective CPP' malarkey. run_phase now returns
the FilePath of its output file; in the event CPP isn't required,
we then just return the input file. (The extra LINE pragma that
the 'ineffective CPP' inserted was a NOP).

Appears to speed things up a bit; mingw/Win32 builds no longer need to
do a slow copyFile operation - other plats are saved from doing a
System.system call.

BTW, there's an opportunity to shorten the critical path for .hs files
some in the driver: at the moment 'unlit' is run over all input.