Install `ghc-gmp.h` C include header file (#9281)

This is mostly interesting when using the in-tree GMP, as there's
no way otherwise to access the in-tree `gmp.h` header file after installation.

In case `integer-gmp2` was build against a system-installed GMP library,
`ghc-gmp.h` simply contains `#include <gmp.h>` for convenience.

Reviewed By: ekmett

Differential Revision: https://phabricator.haskell.org/D522
Persist build-time GMP ver to `HsIntegerGmp.h`

This creates the additional macro definitions in `HsIntegerGmp.h` which
are useful for 3rd party `integer-gmp`-addon libraries.

Here's an example for the definitions created for the in-tree GMP:

  #define GHC_GMP_INTREE     1
  #define GHC_GMP_VERSION_MJ 5
  #define GHC_GMP_VERSION_MI 0
  #define GHC_GMP_VERSION_PL 4
  #define GHC_GMP_VERSION   (5 * 10000 + 0 * 100 + 4)

And here's an example for a system-installed GMP:

  #define GHC_GMP_INTREE     0
  #define GHC_GMP_VERSION_MJ 6
  #define GHC_GMP_VERSION_MI 0
  #define GHC_GMP_VERSION_PL 0
  #define GHC_GMP_VERSION   (6 * 10000 + 0 * 100 + 0)

Part of #9281

Reviewed By: ekmett (via D522)