Don't assume our compiler likes -Wall -Werror
Don't assume our compiler likes -Wall -Werror

and add a -Werror replacement for Solaris Studio.

This only makes Solaris Studio work with the default options, not for
--enable-developer-warnings

Partly addresses #2047

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