Improve forward-compatibility with GNU M4.

Reported by Eric Blake at
<http://lists.gnu.org/archive/html/bug-bison/2008-07/msg00000.html>.
* bootstrap.conf (gnulib_modules): Add unsetenv.
* lib/.gitignore, lib/.cvsignore (/unsetenv.c): Add.
* m4/.gitignore, m4/.cvsignore (/environ.m4): Add.
(/setenv.m4): Add.
* src/output.c (output_skeleton): For the m4 invocation, pass -dV as
the first argument because it may become position-dependent, and unset
POSIXLY_CORRECT so Bison's skeletons have access to GNU M4 extensions.
Add comments explaining these issues in more detail.

m4/.gitignore

@@ -4,6 +4,7 @@
 /dirname.m4
 /dos.m4
 /double-slash-root.m4
+/environ.m4
 /error.m4
 /exitfail.m4
 /extensions.m4
@@ -37,6 +38,7 @@
 /progtest.m4
 /quote.m4
 /quotearg.m4
+/setenv.m4
 /stdbool.m4
 /stdint.m4
 /stdint_h.m4