Appveyor fails

[alphaville]

Describe the bug
Appveyor fails after merging dev/python. The issue seems to be related to bindgen. Perhaps we can copy the appveyor configuration from here.

In particular, with MSVC and target=i686-pc-windows-msvc, we get

thread 'main' panicked at 'Unable to find libclang: "couldn\'t find any valid shared libraries matching: [\'clang.dll\', \'libclang.dll\'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [(C:\\Program Files\\LLVM\\bin\\libclang.dll: invalid DLL (64-bit))])"', src\libcore\result.rs:997:5
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

With target=x86_64-pc-windows-gnu, we get

error occurred: Failed to find tool. Is `gcc.exe` installed? (see https://github.com/alexcrichton/cc-rs#compile-time-requirements for help)

To Reproduce
Check appveyor

Expected behavior
It used to work some time ago.

System information:

• Windows 686
• Windows 32 or 64 + GCC

Additional context