build: Add LIBOQS_NO_VENDOR env variable (#238)

* Allow using LIBOQS_NO_VENDOR env var to force system liboqs

* Document the LIBOQS_NO_VENDOR env variable

README.md

@@ -57,6 +57,9 @@ By default `oqs-sys` attempts to find a system-provided version of `liboqs` and
 falling back to vendored from-source build otherwise.
 You can opt into forcing the vendored build by enabling the `vendored` feature.
 
+Otherwise, if you want to force using the system-provided `liboqs`,
+you can set the `LIBOQS_NO_VENDOR=1` environment variable and the build will fail if the library is not found.
+
 Serde support
 -------------
 

oqs-sys/build.rs

@@ -136,6 +136,9 @@ fn probe_includedir() -> PathBuf {
         return includedir_from_source();
     }
 
+    println!("cargo:rerun-if-env-changed=LIBOQS_NO_VENDOR");
+    let force_no_vendor = std::env::var_os("LIBOQS_NO_VENDOR").map_or(false, |v| v != "0");
+
     let major_version: usize = env!("CARGO_PKG_VERSION_MAJOR").parse().unwrap();
     let minor_version: usize = env!("CARGO_PKG_VERSION_MINOR").parse().unwrap();
     let patch_version = env!("CARGO_PKG_VERSION_PATCH");
@@ -148,7 +151,13 @@ fn probe_includedir() -> PathBuf {
 
     match config {
         Ok(lib) => lib.include_paths.first().cloned().unwrap(),
-        _ => includedir_from_source(),
+        _ => {
+            if force_no_vendor {
+                panic!("The env variable LIBOQS_NO_VENDOR has been set but a suitable system liboqs could not be found.");
+            }
+
+            includedir_from_source()
+        }
     }
 }