Make #define _GNU_SOURCE conditional on C build
Make #define _GNU_SOURCE conditional on C build

I don't completely understand what _GNU_SOURCE does, but without it a
C89 build does not find things like 'environ'.  If you build in C++
it comes up as a duplicate symbol, already included by the compiler.
So I put it conditional under `#ifndef __cplusplus` and it seemed to
work, so that's what it is for now.

draegtun/ren-c,giuliolunati/ren-c,giuliolunati/ren-c,hostilefork/rebol,kealist/ren-c,codebybrett/ren-c,mbk/ren-c,codebybrett/ren-c,mbk/ren-c,codebybrett/ren-c,draegtun/ren-c,giuliolunati/ren-c,draegtun/ren-c,hostilefork/rebol,kealist/ren-c,rgchris/ren-c,codebybrett/ren-c,rgchris/ren-c,draegtun/ren-c,hostilefork/rebol,giuliolunati/ren-c,kealist/ren-c,rgchris/ren-c,hostilefork/rebol,codebybrett/ren-c,rgchris/ren-c,draegtun/ren-c,mbk/ren-c,mbk/ren-c,draegtun/ren-c,giuliolunati/ren-c,rgchris/ren-c,kealist/ren-c,hostilefork/rebol,kealist/ren-c,kealist/ren-c,codebybrett/ren-c,hostilefork/rebol