Tidyup fixes relating to python3 detection and documentation generation.

Tidyup fixes relating to python3 detection and documentation generation.

Fixes a typo and improves the warning configure.ac message if python3
is not found.  Remove gcc/m2/target-independent/*rst files for now.
Provide demangler function for system data types comment.

gcc/ChangeLog:

	* configure.ac (ACX_CHECK_PROG_VER): Improve warning
	message.
	* doc/install.texi (GM2): Update description about
	documentation created if no python3 is detected.

gcc/m2/ChangeLog:

	* Make-maintainer.in (m2-target-independent-doc-rst):
	Disabled.
	* gm2-libs/SYSTEM.def: Corrected typo.
	* target-independent/SYSTEM-pim.texi: (Rebuilt).
	* target-independent/gm2-libs.texi: (Rebuilt).
	* tools-src/def2doc.py (demangle_system_datatype): New
	function.
	(emit_texinfo_content) Call demangle_system_datatype.
	(emit_sphinx_content) Call demangle_system_datatype.
	* target-independent/Builtins.rst: (Removed).
	* target-independent/SYSTEM-iso.rst: (Removed).
	* target-independent/SYSTEM-pim.rst: (Removed).
	* target-independent/gm2-libs.rst: (Removed).

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