configury: ensure m4 is built before calling help2man on it.

Moving the 'doc' related Makefile targets into doc/Makefile.am to
avoid building manual page before the m4 binary itself (suggested
by automake.info).
* Makefile.am (SUBDIRS): Add doc subdir.
* Makefile.am: Move to docs-related rules from here...
* doc/Makefile.am: New file. ...to here.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>