Compatibility with rustup's musl toolchain

[msrd0]

Currently, rlottie-sys will always load libclang.so at build time which is unsupported by rustup's static musl toolchain. To fix this, we need to detect a static-only toolchain and disable the runtime feature of clang/bindgen.

However, this is not currently supported by cargo: https://github.com/rust-lang/cargo/issues/1197

See also #17