Python3 detected in gcc/configure.ac, python3 modules install.texi.

Python3 detected in gcc/configure.ac, python3 modules install.texi.

This patch fixes three problems.  Firstly python3 is now detected
via a re-exp match python3 --version.  Secondly python3 modules
are described in doc/install.texi and modula-2 documentation
dependency upon python3 is mentioned.  Thirdly bug fixes for
--enable-generated-files-in-srcdir with the modula-2 front end
are made to Make-lang.in.

gcc/ChangeLog:

	* gcc/aclocal.m4: Rebuilt.
	* gcc/configure: Rebuilt.
	* gcc/configure.ac (HAVE_PYTHON): Use ACX_CHECK_PROG_VER
	and re-exp to detect Python3.
	* doc/install.texi (GM2): New prerequisite item added.
	(Python3 modules) New item added.
	* m2/Make-lang.in: Bugfixes for
	--enable-generated-files-in-srcdir.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>