Make Happy Explicitly Depend on Perl

[Neil Mitchell]

>> Probably no-one has ever built happy on a machine without perl so this
>> has never been noticed. I think it's more polite to fail at the
>
> More likely, it's because until fairly recently ghc required perl (for the
> Evil Mangler).

I have noticed and reported this in the past. On Windows GHC installs
Perl, but doesn't put it on the %PATH%, which causes building happy to
fail on a default Windows install of GHC.

Thanks, Neil