#ifdef support

It's my intention that we can support Rust compile-time feature enablement based on C++ macros.

Where users are using our `build.rs` support in https://github.com/google/autocxx/tree/main/gen/build this should really be pretty trivial by [printing](https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-cfg) `cargo:rustc-cfg` lines to standard out as we're generating the C++ bindings.

I haven't thought about the exact syntax but obviously the goal is to allow something like `#[cfg(cxx_ifdef = ENABLE_FEATURE)]` which can disable a block of code in Rust.

I also haven't thought about `#if` but that should be achievable in a similar fashion. Probably.

This should be simple for the `build.rs` case but we need to figure out:
* How to do this for the non-Cargo case (https://github.com/google/autocxx/tree/main/gen/cmd) in a way that's easy to integrate into third party build systems. We probably need to write all the extra rustc arguments to a file which build systems can use as input to a subsequent rustc command line.
* How _on earth_ to do this for the integration tests where we just don't launch another rustc instance at all. I don't have a plan there.

I'm also concerned that the sheer number of `#define` symbols are likely to overwhelm the rustc command line so we may need to be selective.

Start by reading the build.rs support under gen/build and the command-line path under gen/cmd, then inspect how integration tests invoke compilation. Define how C++ #define and possibly #if values become Rust cfg inputs for Cargo, third-party build systems, and tests, including whether all symbols can be passed safely. Done means an agreed syntax and working behavior across the supported compilation paths.

Written by the indexing model from the issue text.