makefile: move ghc-options from the cabal file into the makefile.

We shouldn't be hard-coding user preferences into the cabal file that
everyone uses. Instead, GHC should support a variable like CFLAGS that
lets users and developers specify what warnings and optimizations
they would like to use. Instead, we have to fake it: I've standardized
the Gentoo HCFLAGS variable within the makefile, so that a few warnings
will be appended to the (exported) environment variable but otherwise
no flags will be forced by the cabal file.

This should work for me when I'm developing, because the warnings will
be appended to the HCFLAGS="-O2" that I have set in my environment. It
will work on Gentoo, because HCFLAGS are already passed to the cabal
build system in Gentoo. It will work(ish) everywhere else, because
nothing will get passed to the cabal build system, and that's okay.