MFC r267276:

MFC r267276:

  Ensure files are created during the build when using bsd.subdir.mk.

  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.

  This is "make tinderbox" clean.

  Reviewed by:	imp
  Obtained from:	jilles

git-svn-id: svn://svn.freebsd.org/base/stable/10@276422 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f