BUILD: only build the systemd wrapper on Linux 2.6 and above

Attempting to build haproxy-systemd-wrapper on non-linux platforms
sometimes results in build errors. Better move it into an EXTRA
variable which is set to haproxy-systemd-wrapper only on Linux 2.6
and above. Proceeding this way also allows to disable building it
in quick builds (eg: when developing).
1 file changed