dev-lang/ghc: ignore flags passed by cabal with 'QA_CONFIGURE_OPTIONS'

Cabal passes '--with-compiler/--with-gcc' flags to every configure-based hakell project,
but usually they are not used (and not handled).

Reported-by: Patrick Lauer
Gentoo-bug: http://bugs.gentoo.org/515354
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>