gettext 0.11.1 glitch with AC_ISC_POSIX and autoconf 2.53

[Paul Eggert]

I just tried applying "gettextize -c -f" (GNU gettext 0.11.1) to a
copy of diffutils 2.7.10
<ftp://alpha.gnu.org/gnu/diffutils/diffutils-2.7.10.tar.gz>
and I ran into a minor glitch with AC_ISC_POSIX.

diffutils 2.7.10 is already gettextized (with gettext 0.11), so
"gettextize -c -f" doesn't change much, and most of its changes are
OK.  However, I found one minor problem: gettextize added a file
m4/isc-posix.m4 and modified m4/Makefile.am by adding isc-posix.m4 to
EXTRA_DIST.

The glitch here is that diffutils 2.7.10 uses autoconf 2.53, which
does not need the workaround in m4/isc-posix.m4.  That workaround is
not needed in autoconf 2.52f and later, so gettextize need not install
m4/isc-posix.m4 if (say) configure.in contains the line
AC_PREREQ(2.52f) or later.

This is a minor issue, since the extra file (and the unnecessary
change to m4/Makefile.am) don't actually break anything.