* depend.am: Add @AMDEP@ before `include', so that dependency

* depend.am: Add @AMDEP@ before `include', so that dependency
tracking can be disabled.
* depend2.am: Ditto, before all lines related with dependency
tracking.
* m4/depend.m4 (AM_DEP_TRACK): New macro, that documents
--enable/disable-dependency-tracking and sets AMDEP accordingly.
It is implicitly disabled if depcomp is missing.
(AM_DEPENDENCIES): Disable dependency tracking unless it can be
generated by side-effect.
* m4/init.m4 (AM_INIT_AUTOMAKE): Require AM_DEP_TRACK.
* automake.in (handle_dependencies): Add @AMDEP@ before the value
of DEP_FILES.
* m4/depout.m4 (AM_OUTPUT_DEPENDENCY_COMMANDS): Handle DEP_FILES
starting with `#'.