Add modula-2 into doc/install.texi and doc/sourcebuild.texi.

Add modula-2 into doc/install.texi and doc/sourcebuild.texi.

gcc/ChangeLog:

	* doc/install.texi: Add m2 as a language.
	(--disable-libgm2) Documented.  Add make check-m2 to
	testing selected tests.
	* doc/sourcebuild.texi (GM2): New prerequisite item.
        (Python3 modules) New item.  (libgm2) Added.
	(gcc/m2) Added.

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