bsd.cabal.mk: Fix the way documentation is disabled when no Haddock installed.

Reported by:	lev
Integrate from the Ports Collection
bsd.cabal.mk: Improve the fix on haddock detection
bsd.cabal.mk: Move the HADDOCK_AVAILABLE conditional further inside
x11-toolkits/hs-wx: Add missing FILE_LICENSE
x11/hs-xmobar: Add missing FILE_LICENSE
bsd.cabal.mk: Automatize use of PORTDOCS and FILE_LICENSE