correct cohttp opamconstraints

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

Thomas,

Mort just ran into an issue with installing cohttp, as you have to remember to 
specify the ssl package too.  Right now, the cohttp optional dependencies are:

depends: [ "ocamlfind" "re" "uri" {>="1.3.2"} "ounit" ]
depopts: [ "async" {="108.00.02"} "lwt" {>="2.4.1"} "mirage-net" ]

However, cohttp requires Lwt AND SSL to be installed, or it fails (since SSL is 
not yet optional).

Is there any way to express this in the CNF? I tried:

depopts: [ "async" {="108.00.02"} ("lwt" {>="2.4.1"} "ssl") "mirage-net" ]

but that gives me an error.  We cant unconditionally add SSL as that then 
breaks the Mirage/xen build (again, until supported via cross-compilation).

-anil