Going to switch the default Haskell compiler to lang/ghc88

[PHO <pho%cielonegro.org@localhost>]

I'm going to switch the default Haskell compiler from lang/ghc7 to
lang/ghc88, probably this weekend.

> Index: mk/haskell.mk
> ===================================================================
> RCS file: /cvsroot/pkgsrc/mk/haskell.mk,v
> retrieving revision 1.11
> diff -u -r1.11 haskell.mk
> --- mk/haskell.mk	9 Jan 2020 12:29:32 -0000	1.11
> +++ mk/haskell.mk	9 Jan 2020 12:39:18 -0000
> @@ -142,7 +142,7 @@
>  .if ${HASKELL_COMPILER} == "ghc"
>  
>  # Add dependency on GHC.
> -.include "../../lang/ghc7/buildlink3.mk"
> +.include "../../lang/ghc88/buildlink3.mk"
>  
>  # Tools
>  _GHC_BIN=		${BUILDLINK_PREFIX.ghc}/bin/ghc
This will break some of packages that depend on GHC, although I have
already made changes to most of them so they can work both with GHC 7.10
and 8.8. There are packages that simply don't support 8.8 API and need
to be updated to newer versions, so I will update them after switching
the compiler.