Substitute ${SDKROOT} and ${DEVELOPER_DIR} by their environment variables on darwin.

[thb-sb]

Substitute ${SDKROOT} and ${DEVELOPER_DIR} by their environment variables on darwin.

This PR attempts to fix an issue with bindgen when used on darwin with clang
compiled from source (i.e. from @llvm-project).

When using clang compiled from source on darwin, libclang (used through clang-sys) failed
to find system header files (such as stdarg.h or stdbool.h) because these
files are actually living under the SDK path (under <SDK_PATH>/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/ and <SDK_PATH>/usr/include).

Similar to what Bazel did in their xcrunwrapper.sh, we must have a way to tell bindgen's libclang
where to find these system headers.

In this commit, I add the following two substitutes variables:

• ${SDKROOT}: replaced by std::env::var("SDKROOT") if found.
• ${DEVELOPER_DIR}: replaced by std::env::var("DEVELOPER") if found.

This is similar to what it is currently done for ${pwd}.

The following rule should now work on darwin:

// mylib.h
#include <stdarg.h>

# BUILD
cc_library(
    name = "mylib",
    srcs = ["mylib.c"],
    hdrs = ["mylib.h"],
)

rust_bindgen_library(
    name = "mylib_bindgen",
    cc_lib = ":mylib",
    clang_flags = select({
        "@platforms//os:macos": [
            "--sysroot=${SDKROOT}",
            "-isystem${SDKROOT}/usr/include",
            "-isystem${SDKROOT}/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/",
        ],
        "//conditions:default": [],
    }),
    crate_name = "mylib",
    header = "mylib.h",
)

It may also fix #1834.

[illicitonion]

This seems pretty reasonable, thanks! Is it possible to add in a test which would fail without this change?