enabled_if is currently pretty limited. In particular, it is not possible to enable a library depending on run-time configuration as, for instance, determined by dune-configurator.

In the case at hand, we want to optionally compile a library that implements support for OSS sound output. We have this configuration binary:

module C = Configurator.V1

let oss_test = {|
#include <sys/soundcard.h>

int main() {
   return 0;
}
|}

let () =
  C.main ~name:"liquidsoap" (fun c ->
      let oc = open_out "oss_enabled" in
      output_string oc (string_of_bool (C.c_test oss_test));
      close_out oc)

Then this dune configuration:

(library
 (name liquidsoap_oss)
 (libraries liquidsoap_core)
 (enabled_if (= "true" %{read:oss_enabled}))
 (library_flags -linkall)
 (optional)
 (foreign_stubs
  (language c)
  (names oss_io_c))
 (modules oss_io))

But this is currently impossible:

File "src/lib/dune", line 472, characters 25-42:
472 |  (enabled_if (= "true" %{read:oss_enabled}))
                               ^^^^^^^^^^^^^^^^^
Error: Only architecture, system, model, os_type, ccomp_type, profile,
ocaml_version and context_name variables are allowed in this 'enabled_if'
field. If you think that read should also be allowed, please file an issue
about it.