The problem with vmod_debug is that we don't want to distribute it, but
the automake boilerplate unilaterally adds all VMODs to $(vmoddir). This
results in the installation of libvmod_debug.so but not vmod_debug.vcc
and the responsibility of not redistributing libvmod_debug.so to our
downstream maintainers.

A new --noinst option cuts down the amount of circumventions we have for
vmod_debug and ensures that vmod_debug does not accidentally land in
packages.