When FILES is defined in a Makefile that _also_ includes
bsd.subdir.mk, the build of the files (if any) was not properly
triggered during the build stage. This was because bsd.files.mk
did not define the buildfiles target if it was already defined...
and bsd.subdir.mk defined this target on its own, thus causing a
conflict.
Fix this by unconditionally defining buildfiles from bsd.files.mk;
this is safe because nothing else in the tree needs to redefine this
and because the target itself contains no commands: all it does is
define dependencies. Also ensure that bsd.files.mk is always pulled
in by bsd.test.mk regardless of what bsd.prog.mk does.
These fixes allow "make installworld" to run cleanly on a system with
read-only src and obj trees.
Changed based on a patch from jilles.