This may look redundant with `CARGO_FEATURE_<CASE_CONVERTED_NAME>=1`
except that doesn't provide a lossless way of getting the names, e.g. for
forwarding for child builds like tests that need to build examples.
This also makes things more consistent as users
conditionalize on features through `cfg` and this even fits with what
the `CARGO_CFG_` docs say:
> For each configuration option of the package being built, this
> environment variable will contain the value of the configuration, where
> <cfg> is the name of the configuration uppercased and having -
> translated to _. Boolean configurations are present if they are set, and
> not present otherwise. Configurations with multiple values are joined to
> a single variable with the values delimited by ,. This includes values
> built-in to the compiler (which can be seen with rustc --print=cfg) and
> values set by build scripts and extra flags passed to rustc (such as
> those defined in RUSTFLAGS). Some examples of what these variables are:
Fixes#3702
