This improves devnotes.rst with some tricks for Haskell development,
and additionally it does two Makefile improvements:

- properly document lib/_vcsversion.py as a requirement for
  Constants.hs (but do not require rebuild when updated)
- move HEXTRA at the end of the GHC invocation, so any command line
  options will indeed override the built-in ones (especially -osuf)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>