add a configure build option to use system libffi for Linux
ghc should not use its own internal copy of libffi on Linux at least. Copy libs are frowned on by Linux distros. Debian has a patch to use system libffi on ghc-6.12.3 which I am now porting to ghc-7.0.1. I can attach it here but it probably needs to be a build option and/or detected by configure.