Filter invalid CARGO_CFG_ in build scripts

[ehuss]

The environment variables CARGO_CFG_DEBUG_ASSERTIONS and CARGO_CFG_PROC_MACRO are set when a build script is executed. These variables have no meaning, due to the way cfg discovery is done. The build script code should probably avoid setting these two values to avoid any possible confusion.

Cargo has been issuing a warning for a little while for using these in cfg expressions, so I think it should be OK to just filter these two values entirely. cfg values are collected here, so I think just filtering them there at the source should be ok.

[aleksator]

Isn't it better to stop setting those variables altogether instead of filtering them out? If they aren't used at all, that is.
Or are they set by the users of cargo?

[ehuss]

In Cargo's TargetInfo, it collects these cfg values by querying rustc. proc_macro is set because Cargo queries all crate types at once, and proc_macro is included in the list. rustc sets debug_assertions by default, and I don't think it is really necessary to try to unset it with -C debug_assertions=off (that is an option, but it seems easier to just filter it so that these exceptions are listed in one place).

[aleksator]

Got it: those variables are of use on users' side, but shouldn't be on Cargo's side.
Let me do the required change.