declare environ in unistd.h when _GNU_SOURCE feature test macro is used

lots of broken programs expect this, and it's gotten to the point of
being a troubleshooting FAQ topic. best to just fix it.

include/unistd.h

@@ -168,6 +168,7 @@ char *getusershell(void);
 #endif
 
 #ifdef _GNU_SOURCE
+extern char **environ;
 int setresuid(uid_t, uid_t, uid_t);
 int setresgid(gid_t, gid_t, gid_t);
 int getresuid(uid_t *, uid_t *, uid_t *);