I think 4.x is the last FreeBSD who's *.mk work with bmake.
I now only boot-strap bmake  using mk-files as in

./bmake/boot-strap -m `pwd`/mk FORCE_SYS_MK=Generic FORCE_BSD_MK=1


which works on the systems I have access to:
FreeBSD 4.x, 6.x, Linux, Darwin, SunOS

That's with bmake-20071022 and mk-20070430 or later.

On Sun, 9 Dec 2007 17:31:17 -0800, Jan Schaumann writes:
>FreeBSD 6 has a ".WARN dirsyntax" directive in its /usr/share/mk/sys.mk.
>It seems that this causes bmake to bail out after building successfully
>during the test runs: