fixes #5202 -- treat the PYO3_BUILD_EXTENSION_MODULE env var the same as teh extension-module feature

Author: alex

newsfragments/5343.added.md

@@ -0,0 +1 @@
+When building pyo3, setting the `PYO3_BUILD_EXTENSION_MODULE` causes the same effect as the `extension-module` feature. Eventually we expect maturin and setuptools-rust to set this environment variable automatically. Users with their own build systems will need to do the same.

pyo3-build-config/src/impl_.rs

@@ -862,9 +862,14 @@ pub fn get_abi3_version() -> Option<PythonVersion> {
 
 /// Checks if the `extension-module` feature is enabled for the PyO3 crate.
 ///
+/// This can be triggered either by:
+/// - The `extension-module` Cargo feature
+/// - Setting the `PYO3_BUILD_EXTENSION_MODULE` environment variable
+///
 /// Must be called from a PyO3 crate build script.
 pub fn is_extension_module() -> bool {
     cargo_env_var("CARGO_FEATURE_EXTENSION_MODULE").is_some()
+        || env_var("PYO3_BUILD_EXTENSION_MODULE").is_some()
 }
 
 /// Checks if we need to link to `libpython` for the current build target.