bug in libdir setting in ghc mk/config.mk

[Simon Marlow]

> The setting of 'libdir' in "mk/config.mk" in the ghc source 
> is infelicitous. It
> is set using both '=' and ':=' - AFAIK this combination, 
> besides being bad
> practice, does not have a defined meaning.

Thanks, I've fixed this.

Cheers,
	Simon