mk: Add clang specific flag more selectively.

[luqmana]

Only add -Qunused-arguments for clang.

[alexcrichton]

We're currently building successfully on android, so could you describe the error you were seeing in more detail? I'm a little unclear on what the switch here is for.

[luqmana]

@alexcrichton The buildbots work because we don't use --enable-clang on them as far as I'm aware. The reason for my change is when I was building with --enable-clang as well as trying to target android it would pass -Qunused-arguments to arm-linux-androideabi-gcc which doesn't recognize the flag and thus would fail. I believe @pnkfelix was the one who added it originally. I just moved it to only be added to the build triple's C[XX]FLAGS.

[alexcrichton]

So if CFG_BUILD is x86_64-apple-darwin, the cross-compiled code to i686-apple-darwin won't have this flag set, which may mean that it may error?

[luqmana]

Hmmm, well I built with --target=i686-apple-darwin,arm-linux-androideabi and didn't seem to run into errors. I can try just removing it altogether and see if that errors?

[alexcrichton]

We've had some obscure errors in the past from various projects. Sometimes LLVM but perhaps things like libuv as well.

I suppose in theory we should filter per-platform flags and only apply the -Qunused-argument flag to those using clang, although I'm not sure how difficult that would be.

[luqmana]

@alexcrichton I made it add the flag to any target with a clang compiler.

[pnkfelix]

hmm, interesting; maybe the addition of CFG_USING_CLANG was a step in the wrong direction since it sounds like we cannot set that as a global setting anyway, due to cross-compilation scenarios using clang for some contexts and not others.