Check for unsupported system version is broken

[Steven Hartland]

The check added to check and output an unsupported system version is 
broken and instead of output the error, the error errors ;-)
[00:00:01] ====>> Calculating ports order and dependencies
make: "/usr/ports/Mk/bsd.port.mk" line 1164: warning: duplicate script 
for target "show-unsupported-system-error" ignored
make: "/usr/ports/Mk/bsd.port.mk" line 1170: warning: using previous 
script for "show-unsupported-system-error" defined here

The change was added here:
https://svnweb.freebsd.org/ports?view=revision&revision=431746

It's nice to warn the user, but ATM it just looks like someone broke 
something.

     Regards
     Steve