Compile failure with non-cargo build systems

[taiki-e]

The following line fails to compile with non-cargo build systems, like Bazel.

const_fn/src/lib.rs

Line 211 in 9383c4a

const VERSION: Version = include!(concat!(env!("OUT_DIR"), "/version.rs"));

Non-version-based code generation (const_fn(cfg), const_fn(feature)) does not actually need build-scripts, so compile should not fail even if you are using those build systems.
Version-based code generation needs build-scripts, but if used with these build-systems, we should probably always generate normal functions. (This is basically the same behavior as when outputting cfg by build-scripts.)