Log Message:
Various changes to help with KDE3.

      o Enable thread support using pth-syscall and nathanw_sa. The
	later has problems which are being worked on.

      o Restructure the build of certain plugins so that qt3-libs
	comes with jpeg and mng support.

      o Introduce a BUILDLINK_DEPENDS_METHOD.qt3-tools variable that
	when set to build means we don't get a full dependency on
	qt3-tools.