* Makefile.in (lick-fail-on-warning): New rule to compile with warnings

* Makefile.in (lick-fail-on-warning): New rule to compile with warnings
as errors.
(fail-on-warning): Use it.

* lisp/Makefile.in (fail-on-warning): New rule to compile with warnings as
errors.

* dgnushack.el (dgnushack-compile-error-on-warn): New function to call
dgnushack-compile with error-on-warn enabled, and to signal an error if
clean compilation failed.
(dgnushack-compile): New argument 'error-on-warn'.  If non-nil, compile
with `byte-compile-error-on-warn'.  Return nil if errors occured.