Support generation of man pages with help2man

[nirbheek]

This would be implemented via a run_target() that runs a specific built executable on install with help2man which generates a man page. Then we compress it and install it.

So far, pango needs it for its Meson port, but other projects would use it too, I'm sure.

[ebassi]

To be fair, I'd rather port Pango's pango-view utility to docbook and use xsltproc, like we do in GTK+ — but I'm sure other projects would be glad to have help2man support. :-)