rust private

Warn windows-gnu users that the bundled gcc can't compile Add a `DO NOT USE THIS gcc.exe FOR COMPILATION.txt` file to `lib\rustlib\*-pc-windows-gnu\bin` folders in `windows-gnu` installations in order to warn against attempting to use the bundled `gcc.exe` as a C compiler. I'm pretty sure that location is usually found manually, so this should be easily noticeable. This mistake has been made plenty of times and has lead to misunderstandings: Rust: [Bundled gcc (windows x64) is unable to build any c file](https://github.com/rust-lang/rust/issues/24418) gtk-rs: [Compiling on windows](https://github.com/gtk-rs/gtk/issues/625) bzip2-rs: [Build failure at gcc level: blocksort.c not found](https://github.com/alexcrichton/bzip2-rs/issues/30) Alternatives: rename the bundled `gcc.exe` to e.g. `rustc-gcc.exe` or `gcc-linker.exe`. This might require a more comprehensive change or break crates already using it as a linker. r? @alexcrichton