'cabal configure': honor the '--constraint' flag.

Fixes #1150. Fixes #1564.
Merge pull request #1565 from 23Skidoo/configure-constraints

'cabal configure': honor the '--constraint' flag.
When invoking hsc2hs, mention 'cabal_macros.h' only once.

Before & after: https://gist.github.com/23Skidoo/7220763