ports/172741: [ports] check-build-conflicts checks too early for pre-build targets

[arved at FreeBSD.org]

Synopsis: [ports] check-build-conflicts checks too early for pre-build targets

Responsible-Changed-From-To: freebsd-ports-bugs->portmgr
Responsible-Changed-By: arved
Responsible-Changed-When: Wed Nov 21 17:13:33 UTC 2012
Responsible-Changed-Why: 
Looks like bsd.port.mk bug.

But we need a patch!

http://www.freebsd.org/cgi/query-pr.cgi?pr=172741