Adding "Contributing to the MPS" that will show up on GitHub pull requests.
Adding symlink from CONTRIBUTING to contributing.rst to make it easier to find for people used to the GNU conventions.
Fixing grammar problem in MPS contributions agreement.
Adding link to guide.impl.c.format.
Don't define _GNU_SOURCE if already defined.

When using the build method of including the MPS directly into
one's own sources, that build system may already define _GNU_SOURCE
causing a warning here about re-defining it.
Fix typo: it's MPS_BUILD, not MPS_BUILDER.
Merge branch 'pellesc' of github.com:waywardmonkeys/mps-temporary into waywardmonkeys-pellesc
Adding Pelles C as a separate build platform from Microsoft C.
Part of feedback on <https://github.com/Ravenbrook/mps-temporary/pull/2>.
Untested changes.