GUI: Fix build on clang++

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Related to #66 (comment). After changes in #93, when using one of those clang-built kernels, the package.nix would also be built by a clang based stdenv. This gets us closer to having the nix package work with those kernels. However as the gui is currently in the same package, it will also be built by clang. It would probably make sense to split the gui to its own nix package that would then be built by the normal gcc based stdenv which would resolve this, however I don't think it's a bad idea to have the gui build passing on clang anyway. Different compilers have different warnings and all that, helping us catch more mistakes.

I have tested that the nix package (includes driver and GUI) and yeetmousectl still build.

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Rebased to get the new CI workflow

[AndyFilter]

Looks good to me, although I tried to forcibly build it with clang on my Ubuntu and it failed. But I'll blame that on the fact that I'm using a 2 year old version of clang (18.1.3).

I also had to replace -flto=auto with -fuse-ld=lld for it to even try.

But it all doesn't matter, as my system was never meant to run clang in the first place.

Still compiles with g++, and that's all that matters to me, haha.

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@AndyFilter

tried to forcibly build it with clang on my Ubuntu and it failed

Did you do make clean-all && CXX=clang++ make all and it didn't work? I tried on 18.1.8 just now and it did work, but it warned about unknown warning flags. Could be some difference between distros perhaps.

[AndyFilter]

Yea, so turns out I was compiling with clang, just clang, not clang++. All works now (with -fuse-ld=lld).
Kind of makes sense, as all the errors earlier were std-related.

In my defence, the only 2 compilers I've ever used were gcc/g++ and icc/icpx, so I thought that naming was a gcc-type-of thing.