Issues cross-compiling from Linux to Windows

[YaLTeR]

I am cross-compiling from x86-64-unknown-linux-gnu to i686-pc-windows-gnu. Here are some issues I encountered:

• /home/yalter/.cargo/registry/src/github.com-1ecc6299db9ec823/rust_hawktracer_sys-0.4.2/hawktracer/lib/tcp_server.cpp:96:10: fatal error: WinSock2.h: No such file or directory
     96 | #include <WinSock2.h>
        |          ^~~~~~~~~~~~
  

  /home/yalter/.cargo/registry/src/github.com-1ecc6299db9ec823/rust_hawktracer_sys-0.4.2/hawktracer/lib/platform/windows/memory_usage.c:4:10: fatal error: Windows.h: No such file or directory
      4 | #include <Windows.h>
        |          ^~~~~~~~~~~
  

  /home/yalter/.cargo/registry/src/github.com-1ecc6299db9ec823/rust_hawktracer_sys-0.4.2/hawktracer/lib/platform/windows/memory_usage.c:5:10: fatal error: Psapi.h: No such file or directory
      5 | #include <Psapi.h>
        |          ^~~~~~~~~
  

  Fixed by making those includes all-lowercase.

• cargo:rustc-link-search=all=/home/yalter/source/rs/bxt-rs/target/i686-pc-windows-gnu/debug/build/rust_hawktracer_sys-c9794263c9fe5873/out/build/lib/Debug
  

  When using MinGW there's no additional Debug/Release/RelWithDebInfo folder in the end. Fixed by commenting out a line in build.rs.
• A lot of C++ linking errors. Fixed by copying the println!("cargo:rustc-link-lib=dylib=stdc++"); line into the Windows branch as well. Although that made the built .dll require libstdc++-6.dll and a few other MinGW dlls; I couldn't change the line to say static because it couldn't find the static libstdc++ for some reason.

With those hack fixes in place, everything seemed to work fine.

I suspect these issues are specific to windows-gnu in general and not to just cross-compiling.

[AlexEne]

I didn't try this configuration, as I use the msvc windows build.
Would you mind opening a PR with your changes? The CI passes do test windows default config as well as linux, so if it passes, I can merge it. Thinking about it, windows build assumes msvc when build.rs is executed.