Fixup include of gmp/config.mk to use new location

Authored by hvr on Mar 5 2018, 2:13 PM.

Description

Fixup include of gmp/config.mk to use new location

This wasn't spotted rightaway in
8f0b2f5eadf0fcb47c581907205a9db686214a69
because the include-site deliberately ignored include-errors as
a Hack with the justification below:

Hack. The file gmp/config.mk doesn't exist yet after running ./configure in
the toplevel (ghc) directory. To let some toplevel make commands such as
sdist go through, right after ./configure, don't consider this an error.

This may have contributed to Trac #14891.