Export ctru version vars via build script

`rustc-env` only works for the current crate, but plain keys work for
dependent crates via the `DEP_CTRU` keys, which is probably more useful in
general.

ctru-sys/README.md

@@ -13,13 +13,14 @@ to use this library.
 Crate bindings are generated at build time, so the available APIs will depend on the
 installed version of `libctru` when the crate is built. If you want to check
 what version of `libctru` is being built, you can examine these environment
-variables with [`env!`](https://doc.rust-lang.org/std/macro.env.html):
-
-* `LIBCTRU_VERSION`: full version string (e.g. `"2.3.1-4"`)
-* `LIBCTRU_MAJOR`: major version (e.g. `"2"` for version `2.3.1-4`)
-* `LIBCTRU_MINOR`: minor version (e.g. `"3"` for version `2.3.1-4`)
-* `LIBCTRU_PATCH`: patch version (e.g. `"1"` for version `2.3.1-4`)
-* `LIBCTRU_RELEASE`: release version (e.g. `"4"` for version `2.3.1-4`)
+variables from your crate's build script via to its
+[`links` variables](https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key):
+
+* `DEP_CTRU_VERSION`: full version string (e.g. `"2.3.1-4"`)
+* `DEP_CTRU_MAJOR_VERSION`: major version (e.g. `"2"` for version `2.3.1-4`)
+* `DEP_CTRU_MINOR_VERSION`: minor version (e.g. `"3"` for version `2.3.1-4`)
+* `DEP_CTRU_PATCH_VERSION`: patch version (e.g. `"1"` for version `2.3.1-4`)
+* `DEP_CTRU_RELEASE`: release version (e.g. `"4"` for version `2.3.1-4`)
 
 ## License
 

ctru-sys/build.rs

@@ -155,15 +155,16 @@ fn detect_and_track_libctru() {
 
     match get_libctru_version(&pacman) {
         Ok((maj, min, patch, rel)) => {
-            eprintln!("using libctru version {maj}.{min}.{patch}-{rel}");
-
-            // These are accessible by the crate during build with `env!()`.
-            // We might consider exporting some public constants or something.
-            println!("cargo:rustc-env=LIBCTRU_VERSION={maj}.{min}.{patch}-{rel}");
-            println!("cargo:rustc-env=LIBCTRU_MAJOR={maj}");
-            println!("cargo:rustc-env=LIBCTRU_MINOR={min}");
-            println!("cargo:rustc-env=LIBCTRU_PATCH={patch}");
-            println!("cargo:rustc-env=LIBCTRU_RELEASE={rel}");
+            let version = format!("{maj}.{min}.{patch}-{rel}");
+            eprintln!("using libctru version {version}");
+            // These are exported as build script output variables, accessible
+            // via `env::var("DEP_CTRU_<key>")` in other crates' build scripts.
+            // https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key
+            println!("cargo:VERSION={version}");
+            println!("cargo:MAJOR_VERSION={maj}");
+            println!("cargo:MINOR_VERSION={min}");
+            println!("cargo:PATCH_VERSION={patch}");
+            println!("cargo:RELEASE={rel}");
         }
         Err(err) => println!("cargo:warning=unknown libctru version: {err}"),
     }