Missing modules in devel/rust-cbindgen

[bob prohaska]

In trying to compile /usr/ports/devel/rust-cbindgen on an
rpi3 running -current make fails with errors such as

error[E0412]: cannot find type `c_long` in the crate root

It's tempting to think this is some sort of configuration
error. Can anyone suggest a workaround? There don't seem
to be any user-configurable options.

Rust itself now builds successfully on the rpi3, I think
rust-cbindgen might be the only remaining obstacle to
compiling some flavor of firefox.

Thanks for reading, and any guidance!

bob prohaska