editors/openoffice-4: make build more robust

The openoffice build system puts the the ports include directories
(including /usr/local/include) earlier in the list than it's internal
and bundled module include directories. When building on a live
system without poudriere, this can cause the build to pick up unwanted
include files from ports rather than the include files internal to
the build. This can cause strange build failures.

Fix this problem by putting the directories for installed ports at
the end of the list.

Reported by: Jan Henrik Sylvester <me@janh.de>