Commit 6d5227b6 ("Remove obsolete glibc 2.12.1") removed supports for
the separate glibc-ports but also removed GLIBC_USE_PORTS_ADDON. Glibc
versions up to 2.20 bundled support from some architectures in the ports
directory so GLIBC_USE_PORTS_ADDON is required to support these older
glibc versions.
Fixes#1736
Signed-off-by: Chris Packham <judge.packham@gmail.com>