Restore Haskell GLR mode

[andreasabel]

With the hint in haskell/happy#173 (comment), the --glr mode can be tested again.

• Fix generated Makefile to drop happy options --ghc --coerce when --glr is used.
• Add imports needed by GLR-specific code to generated parser Test.

[rachelambda]

Sorry for commenting on this dead issue, but I'm rather confused. The output of bnfc --help (version 2.9.3) states that GLR parsing is deprecated

                --glr                   Output Happy GLR parser [deprecated]

Yet, this issue is closed. Is GLR parsing supported? I was unable to find any mention of this elsewhere.

[andreasabel]

Sorry for the confusion, @depsterr. It is still supported, but deprecated, because it is not supported well upstream (by happy).
In the future, the --glr option might be removed. So far, I am supporting it.