Create man pages for the executables.
* Makefile.am (SUBDIRS): Add `man' and prepend `.' so that
executables be built before help2man is run.
* configure.in (AC_OUTPUT): Add man/Makefile.
AC_MSG_ERROR, not AC_ERROR.
Check for missing help2man.
* man/Makefile.am: New file.
* man/autoconf.x: Likewise.
* man/autoreconf.x: Likewise.
* man/autoheader.x: Likewise.
* man/autoscan.x: Likewise.
* man/autoupdate.x: Likewise.
* man/ifnames.x: Likewise.
* man/common.x: Likewise.
