compat: reinstate support for obsolete $(INCLUDES) variable

compat: reinstate support for obsolete $(INCLUDES) variable

Since INCLUDES and AM_CPPFLAGS can be both used at the same time in a
single project, and interact in a non-obvious way, switching away from
INCLUDES is, unfortunately, not simply a matter of doing a mindless
INCLUDES -> AM_CPPFLAGS substitution.  In fact, the transition from a
"INCLUDES + CPPFLAGS" setup to a "AM_CPPFLAGS only" setup could cause
subtle bugs to sneak into the build systems of the client packages
doing the transition.  Forcing this risk upon our users is definitely
not worth the small enhancement in consistency and cleanliness that
the removal of INCLUDES would give us.

For a more extended rationale and related discussions, see:
<http://lists.gnu.org/archive/html/automake/2013-02/msg00001.html>

This change basically reverts commit v1.13-8-g880595d.

* automake.in: Re-introduce support for INCLUDES (its use will cause
warnings in the 'obsolete' category rather than fatal errors).
* doc/automake.texi: Adjust to re-add mention of INCLUDES.
* t/includes-no-more.sh: Rename ...
* t/includes-deprecation.sh: ... like this, and adjust.
* t/list-of-tests.mk: Update.
* t/warnopts.sh: Minor fixlet while at it.

Suggested-by: Peter Rosin <peda@lysator.liu.se>
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>