Latest release breaks cross compiling (target Mac OS)

[Edu4rdSHL]

Seems that latest backtrace-sys version is broken for cross compiling (MacOS target). It's what I got when trying to compiling for Mac OS:

https://gist.github.com/Edu4rdSHL/49b8b52bac916d88e32861a817d5f9a5

[Edu4rdSHL]

Fixed adding: CC=o64-clang CXX=o64-clang++ cargo build --release --target=x86_64-apple-darwin thanks.