libstdc++: Regenerate <bits/version.h> in maintainer mode

libstdc++: Regenerate <bits/version.h> in maintainer mode

This is a generated header but regenerating it requires the manual step
of running 'make -C include update-version' in the libstdc++ build dir.
Make it regenerate automatically when --enable-maintainer-mode is used.

libstdc++-v3/ChangeLog:

	* include/Makefile.am [MAINTAINER_MODE]: Add target to
	automatically update <bits/version.h>.
	* include/Makefile.in: Regenerate.

gcc/ChangeLog:

	* doc/install.texi (Prerequisites): Document use of autogen for
	libstdc++.