Expose control of import prefix to Cargo builds

[dtolnay]

This PR adds a global configuration object called cxx_build::CFG.

• CFG.include_prefix

Presently the only exposed configuration is the include_prefix, the prefix at which C++ code from your crate as well as directly dependent crates can access the code generated during this build.

By default, the include_prefix is equal to the name of the current crate. That means if our crate is called demo and has Rust source files in a src/ directory and maybe some handwritten C++ header files in an include/ directory, then the current crate as well as downstream crates might include them as follows:

  // include one of the handwritten headers:
#include "demo/include/wow.h"

  // include a header generated from Rust cxx::bridge:
#include "demo/src/lib.rs.h"

By modifying CFG.include_prefix we can substitute a prefix that is different from the crate name if desired. Here we'll change it to "path/to" which will make import paths take the form "path/to/include/wow.h" and "path/to/src/lib.rs.h".

// build.rs

use cxx_bridge::CFG;

fn main() {
    CFG.include_prefix = "path/to";

    cxx_build::bridge("src/lib.rs")
        .file("src/demo.cc") // probably contains `#include "path/to/src/lib.rs.h"`
        /* ... */
        .compile("demo");
}

Note that cross-crate imports are only made available between direct dependencies. Another crate must directly depend on your crate in order to #include its headers; a transitive dependency is not sufficient. Additionally, headers from a direct dependency are only importable if the dependency's Cargo.toml manifest contains a links key. If not, its headers will not be importable from outside of the same crate.