Error when no rst2man is found
Error when no rst2man is found

If one runs configure with --without-rst2man or no rst2man is found,
the value is "no", not an empty string.  Handle this correctly in
configure.

Fixes: #1473

gquintard/Varnish-Cache,gauthier-delacroix/Varnish-Cache,gquintard/Varnish-Cache,franciscovg/Varnish-Cache,mrhmouse/Varnish-Cache,feld/Varnish-Cache,alarky/varnish-cache-doc-ja,alarky/varnish-cache-doc-ja,franciscovg/Varnish-Cache,alarky/varnish-cache-doc-ja,chrismoulton/Varnish-Cache,chrismoulton/Varnish-Cache,zhoualbeart/Varnish-Cache,mrhmouse/Varnish-Cache,varnish/Varnish-Cache,zhoualbeart/Varnish-Cache,feld/Varnish-Cache,gauthier-delacroix/Varnish-Cache,zhoualbeart/Varnish-Cache,gauthier-delacroix/Varnish-Cache,ajasty-cavium/Varnish-Cache,gauthier-delacroix/Varnish-Cache,zhoualbeart/Varnish-Cache,gquintard/Varnish-Cache,varnish/Varnish-Cache,chrismoulton/Varnish-Cache,franciscovg/Varnish-Cache,mrhmouse/Varnish-Cache,varnish/Varnish-Cache,mrhmouse/Varnish-Cache,ajasty-cavium/Varnish-Cache,franciscovg/Varnish-Cache,feld/Varnish-Cache,ajasty-cavium/Varnish-Cache,chrismoulton/Varnish-Cache,gquintard/Varnish-Cache,ajasty-cavium/Varnish-Cache,feld/Varnish-Cache,franciscovg/Varnish-Cache,gauthier-delacroix/Varnish-Cache,varnish/Varnish-Cache,alarky/varnish-cache-doc-ja,alarky/varnish-cache-doc-ja,varnish/Varnish-Cache,zhoualbeart/Varnish-Cache,ajasty-cavium/Varnish-Cache,feld/Varnish-Cache,chrismoulton/Varnish-Cache,mrhmouse/Varnish-Cache