Add support for cross compilation

[trobanga]

In order to cross compile to Android I had to adjust miniaudio-sys/build.rs by adding clang_arg("-Ipath/to/android/toolchain/.../sys-root/usr/include") to bindings and manually set ma_android = true as target_os is equal to the host system.

Are you planning on adding cross compilation support in the future?

[ExPixel]

Sure, I don't see why not. I've been holding off on updates because there seem to be plans for a 1.0 version of miniaudio in the works so when that happens I'll incorporate your change as well :)

[trobanga]

Ok, great! I'll send you a working version for Android and iOS once I have it.