meson: add custom targets man/man and man/html

meson: add custom targets man/man and man/html

This provides functionality similar to the ./configure --disable-manpages
switch. Man pages are built by default (if xsltproc is found), html pages
are not. Those default can be changed with -Dman=no, -Dhtml=yes/auto.
It is still possible to build one or the either, even if not configured, with
ninja-build man/man and ninja-build man/html.

v2:
- obey conditionals in index/directives list