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.

doc/bison.1

@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man 1.36.
-.TH BISON "1" "May 2008" "bison 2.3b+" "User Commands"
+.TH BISON "1" "July 2008" "bison 2.3b+" "User Commands"
 .SH NAME
 bison \- GNU Project parser generator (yacc replacement)
 .SH SYNOPSIS
@@ -174,6 +174,8 @@ disable the report
 Written by Robert Corbett and Richard Stallman.
 .SH "REPORTING BUGS"
 Report bugs to <bug\-bison@gnu.org>.
+.PP
+Id
 .SH COPYRIGHT
 Copyright \(co 2008 Free Software Foundation, Inc.
 .br