They cause the following warnings:
warning: attribute must be of the form `#[link(name = "...", /*opt*/ kind = "dylib|static|...",
                                               /*opt*/ cfg = "...")]`
 --> /tmp/coreaudio-sys/target/x86_64-apple-darwin/debug/build/coreaudio-sys-0d8fd082e359c2ec/out/coreaudio.rs:3:1
  |
3 | #[link = "/tmp/MacOSX10.14.sdk/System/Library/Frameworks/AudioToolbox"]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(ill_formed_attribute_input)] on by default
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #57571 <rust-lang/rust#57571>

And they are actually redundant with the cargo:rustc-link-lib lines the
build script prints already.

The current setup requires 2 environment variables for
cross-compilation, and can fail to build under some circumstances on a
freshly installed OSX + Xcode machine.

This change simplifies the setup by:
- On native builds, using `xcrun --show-sdk-path` instead of
  `xcode-select -p` to find the SDK.
- On cross builds, checking COREAUDIO_SDK_PATH.
- Pass --target to clang, which makes things work for cross-compilation
  with plain clang (as opposed to osxcross).
- Pass -isysroot to clang instead of -F, which makes it find all
  possible headers in the SDK, especially TargetConditionals.h, which
  lives in $COREAUDIO_SDK_PATH/usr/include.
- Instruct cargo of the environment variables that affect the build
  script.

COREAUDIO_CFLAGS is left as a convenience, for now.

As of 0.49, bindgen supports a BINDGEN_EXTRA_CLANG_ARGS environment variable
that can be used to pass extra arguments to clang. This can be used
instead of COREAUDIO_CFLAGS.

Because recent versions of bindgen generate their bindings on one line,
because of bindgen's use of u128 and rust's warnings of its unsafety
FFI-wise, and because of rust-lang/rust#62999,
combined, building the crate generates massive logs... which exceed
travis's maximum log length.

It was disabled in RustAudio#13 because back then rustfmt had some problems with
the bindgen output. Now that rustfmt is part of rust, these kind of
problems presumably should happen less if at all.

Simply printing errors to stderr does nothing visible by default. So
when building without an SDK available, there is no apparent error
showing up other than in some file "hidden" in target/, and an error
that including coreaudio.rs failed.

Similarly, when attempting to build for an unsupported target, the
script reaches the `unreachable!()`.

Instead of passing the full paths to the headers to bindgen, create a
meta header that #include's all the needed ones. This leaves it to clang
to find them in the right /System/Library/Frameworks directory.

Now that the build script itself doesn't need the SDK path, allow not to
specify one explicitly. This allows things to work out when the
environment works out of the box one way or another (via wrapper
scripts, setting BINDGEN_EXTRA_CLANG_ARGS, etc.)