Build script access environment at compile time instead of at build time

[redsun82]

The build script uses env!("CARGO_PKG_VERSION"), relying on that variable being set by cargo when the build script is compiled. However, the docs state that inputs to the build script are passed at run time (of the build script):

When the build script is run, there are a number of inputs to the build script, all passed in the form of environment variables.

(emphasis mine)

In particular, that is for example what the bazel wrapper around rust rules expects, so that any crate depending on this one will fail to build with bazel.

[redsun82]

Some time ago I've opened #17. I'm currently using that as a workaround, adding to our Cargo.toml (that is used as a base for generating the bazel build configuration) this snippet:

[patch.crates-io]
# patch for build script bug preventing bazel build
# see https://github.com/rust-lang/rustc_apfloat/pull/17
rustc_apfloat = { git = "https://github.com/redsun82/rustc_apfloat.git", rev = "32968f16ef1b082243f9bf43a3fbd65c381b3e27" }