Warn windows-gnu users that the bundled gcc can't compile

[ljedrz]

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
gtk-rs: Compiling on windows
bzip2-rs: Build failure at gcc level: blocksort.c not found

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

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[alexcrichton]

Thanks! I wonder though if perhaps we could start out with a README.txt file or something like that? If that doesn't act as enough deterrent we can go I think with the "alarming filename"

[ljedrz]

I've thought about it, but it might not be visible enough - when someone finds gcc.exe they add that location to their PATH and later, when they receive a misleading error, they are not investigating GCC, but rather the affected library. The name could be a bit less aggressive and still arising suspicion, though - maybe GCC WARNING.txt?

[alexcrichton]

Ok! Although perhaps a dash in the middle as I'm not sure I trust out shell scripts to handle spaces in filenames :(

[alexcrichton]

@bors: r+

[bors]

📌 Commit 232e77e has been approved by alexcrichton

[bors]

⌛ Testing commit 232e77e with merge 77117e3...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 77117e3 to master...