isnan: port to VAX

Reported by John Klos for NetBSD-5/VAX in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-10/msg00133.html>.
* lib/isnan.c (IEEE_FLOATING_POINT): New macro, stolen from Emacs.
(FUNC): Use it.
maint.mk: prefer gpgv2 over gpgv

* top/maint.mk (gpgv): Use gpgv2 if present, else gpgv.
(gpg_key_ID): Use $(gpgv), rather than hard-coding "gpgv".
Reported by Gary Vaughan.
obstack: pacify HP C

* lib/obstack.h (obstack_free) [!__GNUC__]: Rewrite to avoid
warning "conversion from pointer to smaller integer" from HP
C-ANSI-C - cc version B9007AA/B3910B A.06.26.  It's safe to assume
C89 or later nowadays, so cast to void instead of int.  Privately
reported by H.Merijn Brand.  Also, change header to match glibc's,
to make checking against glibc easier.