Get rid of custom setup and `cabal-doctest` dependency

[andreasabel]

The custom setup in BNFC.cabal causes installation of the bnfc executable to require Cabal, which can take long to build.

bnfc/source/BNFC.cabal

Lines 70 to 74 in 220e985

	custom-setup
	setup-depends:
	base >= 4 && <5,
	Cabal >= 2.0,
	cabal-doctest >= 1 && < 1.1

This is regrettable, since such a setup is only necessary as a means to enable doctest, yet a cabal install does not even provide means to run testsuites.
We should look into other ways than cabal-doctest to build the doctest driver.