Provide the include directory to consumers

For users using both C/C++ and Rust, it's useful to have access
to the mimalloc include directory in the build-script (of crates
directly depending on `libmimalloc-sys`). This allows using the
header in user build-scripts, without needing to vendor the mimalloc header.
This mechanism is also already used in crates like libz-sys, see
https://github.com/rust-lang/libz-sys/blob/80c597a07f5e5f0dff0e98c03d48a77845de9f6e/build.rs#L171

See <https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key>
for documentation.

Author: jschwe

libmimalloc-sys/build.rs

@@ -1,4 +1,5 @@
 use std::env;
+use std::path::Path;
 
 fn main() {
     let mut build = cc::Build::new();
@@ -9,6 +10,18 @@ fn main() {
         "v2"
     };
 
+    let cargo_manifest_dir = env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR not set");
+    let include_dir = Path::new(&cargo_manifest_dir)
+        .join("c_src/mimalloc/")
+        .join(version)
+        .join("include")
+        .to_str()
+        .expect("include path is not valid UTF-8")
+        .to_string();
+    // Make the include directory available to consumers via the `DEP_MIMALLOC_INCLUDE_DIR`
+    // environment variable.
+    println!("cargo:INCLUDE_DIR={include_dir}");
+
     build.include(format!("c_src/mimalloc/{version}/include"));
     build.include(format!("c_src/mimalloc/{version}/src"));
     build.file(format!("c_src/mimalloc/{version}/src/static.c"));