Make RST output directory configurable as well.
Make RST output directory configurable as well.

Avoid some of the mess in src/ when building multiple vmods
in the same git tree.

feld/Varnish-Cache,varnish/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,varnish/Varnish-Cache,varnish/Varnish-Cache,varnish/Varnish-Cache,feld/Varnish-Cache,varnish/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache