Support incremental builds with CMake

[Patryk27]

When CMake builds a project, it generates a file called CMakeCache.txt that contains - among others - a dump of environmental variables.

This file is generated both for the deps-only- and the main- derivation, and if env-vars happen to differ between both¹, CMake will refuse to compile the latter derivation, saying:

CMake Error: The current CMakeCache.txt directory ... is different than the directory ... where CMakeCache.txt was created.

Solving this problem is easy - one just has to remove CMakeCache.txt to let CMake regenerate it.

Note that the original bug report (linked below) suggests to use sed, but that doesn't work on Darwin which uses a different convention for temporary paths - compare error messages for the failing derivations:

... on Linux:

CMake Error: The current CMakeCache.txt directory /build/source/target/release/build/oqs-sys-a19e222d9ef825d3/out/build/CMakeCache.txt is different than the directory /build/dummy-src/target/release/build/oqs-sys-a19e222d9ef825d3/out/build where CMakeCache.txt was created. This may result in binaries being created in the wrong place. If you are not sure, reedit the CMakeCache.txt

... and on Darwin:

CMake Error: The current CMakeCache.txt directory /tmp/nix-build-app-0.1.0.drv-5/source/target/release/build/oqs-sys-4b5e25b1671aa1e5/out/build/CMakeCache.txt is different than the directory /tmp/nix-build-app-deps-0.1.0.drv-0/source/target/release/build/oqs-sys-4b5e25b1671aa1e5/out/build where CMakeCache.txt was created. This may result in binaries being created in the wrong place. If you are not sure, reedit the CMakeCache.txt

As for the implementation, since two-step builds are a Naersk-specific construct, I think solving this transparently on Naersk's side instead of just adding a tip into README.md is the better approach here, since it keeps Naersk plug-and-play'able.

Closes #285.

¹ which is not that difficult, e.g. that's how pkgs.rustPlatform.bindgenHook behaves

[nmattia]

Missed the notification! I'll have a look ASAP

[Patryk27]

Ping, @nmattia 🙂

If you don't have time, it's alright, of course - I'm re-pinging just in case.

[nmattia]

@Patryk27 sorry, completely dropped the ball 😬 Lots going on privately

Is there any way you could split out the bits not directly related to CMake (formatting, package updates, etc)? That'd make it easier to review, right now I'm not 100% sure what's needed by the "CMake changes" and was is just clean up

[Patryk27]

Sure, sure - that exists as a single commit only because it's required for the test to exist - I'll split it into separate commits when I find a moment 🙂

[Patryk27]

Okie, commit split 🙂

[nmattia]

Thanks a lot! Guess this had very little to do with CMake in the end :)

[nmattia]

@Patryk27 sorry for the delay! Bit snowed under with personal stuff at the moment, but don't hesitate to ping me again if I forget :)

[Patryk27]

Ping, ping @nmattia - should be much easier to review now 😄

I've had to bump Nix sources and add Fenix to get tests working on aarch64, but other than that, there's only the README.md change.

[nmattia]

Sorry for the delay, I was out for a bit! Let's gooo :)