meson support for clang broken

[Ralith]

meson's setup-hook.sh sets CC=@cc@/bin/cc CXX=@cc@/bin/c++, clobbering the values provided by the stdenv. This (silently?) breaks clangStdenv and, because gcc doesn't support -stdlib=libc++, noisily breaks libcxxStdenv.

This was introduced by 284f75a (cc @shlevy). Never having touched the crosscompilation infrastructure before, I'm not sure what the intended behavior here is, let alone the correct way to do it, but it probably shouldn't be forcing everything to use some arbitrary gcc.

[Ralith]

On closer examination I see that what's happening is that the meson package hardcodes the stdenv it's built with, therefore breaking use targeting any other stdenv. Was this intended?

[shlevy]

Not necessarily intended, but not sure the nicest solution either. Maybe a way to bundle meson with a particular stdenv? @Ericson2314 , thoughts?

[Ericson2314]

Can we just not set those vars? The environment should already have them.

[shlevy]

There was definitely a real case where this was needed (cross systemd maybe?) but I don't remember the symptoms.

[Ericson2314]

I can't even find where cc is being set. https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/tools/build-managers/meson/default.nix#L54-L70 I'd hope would help too.

Coincidentally I also opened mesonbuild/meson#3282, a while back, which I'm guessing would be a better solution to whatever prompted @shlevy to add those vars.

[Ralith]

I assume cc is being set at https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/tools/build-managers/meson/default.nix#L73 and substituted in by default somewhere.

[stale]

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[Ericson2314]

This was fixed in 3c00ca0#diff-6b138c4aaf5164baf820a221e2d6dd93 :)

[nh2]

@Ericson2314 Thanks, that was sorely needed.

Fixes my non-cross C++ builds with Meson.

[L-as]

I can't seem to use overrideCC stdenv llvmPackages_10.lldClang as my stdenv still.
meson gives me this:

Using 'CC_FOR_BUILD' from environment with value: 'gcc'
Using 'AR_FOR_BUILD' from environment with value: 'ar'
Using 'CXX_FOR_BUILD' from environment with value: 'g++'

meson.build:21:0: ERROR: Compiler g++ can not compile programs.

[L-as]

Made a new issue, since it only seems to be mesa?