MFC 288829

MFC 288829

  The latest version of lex requires the latest m4 to build, add a dependency
  when running the build-tools stage.

  The requirement is due to the -P flag used when running m4 from usr.bin/lex
  Makefile to generate skel.c.  With the old m4 that fails and the failure is
  ignored, resulting in an empty(-ish) skel.c, which leads to later build
  failures when the misconfigured new lex tool is run.

  This enables building -current (and 10-stable after MFC) on a stable-8
  system again.

git-svn-id: svn://svn.freebsd.org/base/stable/10@289145 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f