Changes to support bootstrapping the compiler from .hc files.  It's
not quite working yet, but it's not far off.

  - the biggest change is that any injected #includes are now placed in
    the .hc file at generation time, rather than compilation time.  I
    can't see any reason not to do this - it makes it clear by looking at
    the .hc file which files are being #included, it means one less
    temporary file at compilation time, and it means the .hc file is more
    standalone.

  - all the gruesomeness is in mk/bootstrap.mk, which handles building
    .hc files without a ghc driver.