Deadlock when calling pthread_key_create in the destructor of a pthread_key

[王宇逸]

> Rust does not support Cygwin, and gcc-rs is not yet available on Cygwin: what 
> tools and libraries were used to build this library and utilities?

Hello Brian, I'm happy to announce the cygwin target support in rust, which is
maintained by myself now:) Now it provides std support but no host tools support.
You can try it with latest nightly toolchain (either windows-gnu or windows-msvc)
and cross-compile to cygwin.

$ cargo build --target x86_64-pc-cygwin -Zbuild-std

Back to the question, I have made the std *almost* work on cygwin, but there might
be some bugs. I tried to run the unit tests and found this bug.