B0_ocaml.MetaSourceMetadata keys
c_requires hack for now this simply passes these options as -cclib options.
requires on a build unit specifies the OCaml libraries needed to compile a unit.
library on a build unit specifies that the unit defines the library with the given name.
FIXME quick hack this should not be in meta.
supported_code indicates which backend code the unit supports. If the meta is unspecified this assumes `All.
needs_code indicates the unit unconditionally needs a given code build.