rpm: support old Fedora/CentOS autoconf

--runstatedir option was introduced in autoconf >= 2.70
but all RPM platforms are currently at 2.69 which resulted in error:

    configure: error: unrecognized option: `--runstatedir=/run/bird`

Use equivalent --with-runtimedir option instead which works with older
autoconf.

Once some RPM platforms ship autoconf >= 2.70, a %if RPM macro can be
used to select appropriate option per-distro.