Portage/GHCCore.hs: take into account user specified cabal flags when guessing minimum ghc version

Otherwise we can pick something, that binds to only old package versions.
Like it was in snap-0.13.2.0 (--flag=old-base required outdated lens-3.8)

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>