pkg.m4 breaks if 'version' is defined as a macro

If the user defines an m4 macro called version, pkg.m4 outputs a broken configure script due to underquoting in PKG_PROG_PKG_CONFIG:

$ cat >configure.ac <<EOF
> AC_INIT([Test], [0.1.0])
> m4_define([version], [DEFINITION-OF-VERSION])
> PKG_PROG_PKG_CONFIG
> AC_OUTPUT
> EOF
$ autoreconf -fi
$ ./configure
checking for pkg-config... /usr/bin/pkg-config
checking pkg-config is at least DEFINITION-OF-VERSION 0.9.0... pkgconf: unknown option -- atleast-pkgconfig-DEFINITION-OF-VERSION
no
configure: creating ./config.status

(This example was run on a system that uses pkgconf and not pkg-config, but pkg.m4 is shared between the two.)