Log Message:
Fix serious bug where BUILDLINK_PACKAGES wasn't being ordered properly
by moving the inclusion of buildlink3.mk files outside of the protected
region.  This bug would be seen by users that have set PREFER_PKGSRC
or PREFER_NATIVE to non-default values.

BUILDLINK_PACKAGES should be ordered so that for any package in the
list, that package doesn't depend on any packages to the left of it
in the list.  This ordering property is used to check for builtin
packages in the correct order.  The problem was that including a
buildlink3.mk file for <pkg> correctly ensured that <pkg> was removed
from BUILDLINK_PACKAGES and appended to the end.  However, since the
inclusion of any other buildlink3.mk files within that buildlink3.mk
was in a region that was protected against multiple inclusion, those
dependencies weren't also moved to the end of BUILDLINK_PACKAGES.