add vmod-std to the build

[Tollef Fog Heen]

]] Andrea Campi 

| Also, eventually it would be nice not to require a checked out tree.

This is currently because we don't promise anything wrt API and ABI
stability.  Eventually, I'm hoping we can have a stable API and ABI, but
given how new the vmod mechanism is, we don't know if we've gotten it
right or not yet.

| The tricky part is doing it in such a way that e.g. vmod.py emits correct
| includes when building in-tree (e.g. #include "bin/varnishd/cache.h") and
| out (e.g. #include <varnish/cache.h> or include <varnish/private/cache.h>
| or something like that).

That's one of the easier problems to solve, really.

| I guess it all depends on how private you want to keep some of the header
| files.

Yup, basically.

Cheers,
-- 
Tollef Fog Heen
Varnish Software
t: +47 21 98 92 64