Build outside of source dir (using cmake crate)

[jonhoo]

Locally I also need linux-rdma/rdma-core#1485 for this to work.

[jonhoo]

sigh as far as I can tell this is failing because the path we're building in is now too long to fit inside of sun_path (the path part of UNIX domains sockets ..

[jonhoo]

Ref https://unix.stackexchange.com/questions/367008/why-is-socket-path-length-limited-to-a-hundred-chars

[jonhoo]

Fixed by making the target directory be at /tmp/cargo instead.

The move to Clang is probably actually unnecessary now, but 🤷

[jonhoo]

Also worth noting that this change means we also run cmake --build (not just cmake), which I think is technically unnecessary to get the input we need for bindgen. Hoping that won't cause major headache for folks. There unfortunately doesn't appear to be a way to make the cmake crate not call --build.

[jonhoo]

Filed a ticket asking for this functionality in the cmake crate: rust-lang/cmake-rs#207

[jonhoo]

Also, FYI, after I change the invocation to not also call the install target I no longer need linux-rdma/rdma-core#1485 (since we never hit that step).

[jonhoo]

Released in ibverbs-sys 0.2.1 🎉