Sylvain Henry authored Feb 07, 2020 and Marge Bot committed Feb 18, 2020

* Hadrian doesn't use integer-gmp/config.mk file anymore to determine if
  building GMP in-tree is required.

  "config.mk" is created by Cabal when the integer-gmp package is
  configured and this file is still untracked by Hadrian. This led to a
  tricky configure "race" because "config.mk" is built by the
  "setup-config" rule, but this rule is also used to find dependencies,
  in particular the "ghc-gmp.h" header, but the creation of this file
  was depending (without being tracked) on "config.mk".

  Now Hadrian only builds in-tree GMP if `--with-intree-gmp` is passed
  to the top-level configure script.

* in-tree GMP isn't built once for all in a fixed stage (Stage1)
  anymore. It is built per stage which is required if we build a
  cross-compiler

* switching between in-tree and external GMP is now supported without
  having to clean the build directory first.

* "wrappers.c" now includes "ghc-gmp.h" instead of "ghc.h". It
  helps ensuring that the build system generates "ghc-gmp.h".

* build in-tree GMP in "<root>/stageN/gmp/gmpbuild" and produce useful
  artefacts (libgmp.a, gmp.h, objs/*.o) in "<root>/stageN/gmp"