Make #if conditions in PSM compliant with CPP

[adsnaider]

Not a problem when building with cargo proper but when we are using rules_rust in bazel, it uses our C toolchain which we use with -Wundef and -Werror, causing a compilation failure. This fixes the issue by using the defined preprocessor statement (https://gcc.gnu.org/onlinedocs/cpp/Defined.html)

[nagisa]

Thank you for contributing. This change still needs test coverage.

[adsnaider]

Sure. @nagisa what are you hoping to test exactly?

[nagisa]

Well, if there are conditions under which the build fails, then a test added here would have to be failing before this change and passing after it. This probably can be as simple as somehow forcing -Wundef -Werror in one/all of the gh workflows, possibly via a CFLAGS or similar env var that the cc crate looks at.

[adsnaider]

Done. First commit triggers the failure in the test and second commit fixes it. Unfortunately it only tests the failure is corrected in x86_64-unknown-linux-gnu because I couldn't figure out a way to set CFLAGS more broadly on all non msvc builds (aside from copying the CFLAGS_* line for a bunch of targets). I could modify it in the build.rs but that seems somewhat worse and if I put the flag in CI then the local vs. CI test behavior diverges. Happy to change to a different approach though

[adsnaider]

@nagisa anything else you need here?

[nagisa]

Just to find a little free time to work on this crate.

I couldn't figure out a way to set CFLAGS more broadly on all non msvc builds (aside from copying the CFLAGS_* line for a bunch of targets

Would setting the CFLAGS environment variable in .github/worklows not set the flag universally? I guess for Windows the flags might use a different format, but then we have matrices for different targets there.