Try to improve Make dependency for `ghc-gmp.h`

This will hopefully workaround an issue where
`libraries/integer-gmp2/include/ghc-gmp.h` gets deleted
during cleanup but isn't regenerated.

This situation is caused by `./validate` cleaning the tree with

  $make maintainer-clean NO_CLEAN_GMP=YES

which doesn't doesn't clean the `gmp/` folder, and so the `include/ghc-gmp.h`
file wasn't recreated (as it was previously a side-effect of building
the intree GMP).

Reviewed By: luite

Differential Revision: https://phabricator.haskell.org/D523
Use the `patch` command detected by the top-level `configure`

...instead of invoking `patch` directly in `integer-gmp2/gmp/ghc.mk`