[ng] compile: don't support $(INCLUDES) anymore, it's obsolete

[ng] compile: don't support $(INCLUDES) anymore, it's obsolete

The $(INCLUDES) variables has been since long supplanted and obsoleted
by the $(AM_CPPFLAGS) variable (in documentation since the commit
3d248ab5 of 2001-06-12, "* automake.texi (Program variables): Document
AM_CPPFLAGS. Deprecate INCLUDES", and in code since the commit '1415d22f'
of 2002-07-09, "* automake.in (generate_makefile): Suggest using
AM_CPPFLAGS instead of INCLUDES".

So we can drop support for $(INCLUDES).

* NG-NEWS, doc/automake-ng.texi: Update.
* automake.in: Drop usages of $(INCLUDES) from several calls to
'register_language'.
(generate_makefile): Don't explicitly warn about the definition of
the $(INCLUDES) variable anymore.
* t/pluseq5.sh: Adjusted to avoid using the INCLUDES variable.
* t/werror2.sh: Likewise.
* t/warnopts.sh: Likewise, and remove extra-verbose comment.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>