Fix: standardize man pages building/installing

This patch makes the build system act as follows:

    if --enable-man-pages:
        if in Git repo:
            require asciidoc/xmlto (configure)
            build man pages (make, make clean)
            install man pages (make install)
            distribute man pages (make dist)
        else:
            if asciidoc/xmlto both exist:
                build man pages (make, make clean)
                install man pages (make install)
                distribute man pages (make dist)
            else:
                warn that asciidoc/xmlto are missing (configure)
                create "error" man page targets in Makefile (make)
                do not clean man pages (make clean not available)
                install distributed man pages (make install)
                distribute distributed man pages (make dist)
    else if --disable-man-pages:
        do not build man pages (make, make clean not available)
        do not install man pages (make install not available)
        do not distribute man pages (make dist not available)

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>