pyo3-build-config: Build "abi3" extensions without an interpreter

Ignore the build host Python interpreter configuration when
compiling portable "abi3" extension modules.
Such extensions should not depend on a concrete Python version
or build configuration by design.

Maturin already does this when building "abi3" extension wheels
by defining `PYO3_NO_PYTHON=1` for Cargo when an `abi3-py3*`
feature is detected.

Closes #2292

pyo3-build-config/src/impl_.rs

@@ -674,14 +674,14 @@ pub fn is_extension_module() -> bool {
 
 /// Checks if we need to link to `libpython` for the current build target.
 ///
-/// Must be called from a crate PyO3 build script.
+/// Must be called from a PyO3 crate build script.
 pub fn is_linking_libpython() -> bool {
     is_linking_libpython_for_target(&target_triple_from_env())
 }
 
 /// Checks if we need to link to `libpython` for the target.
 ///
-/// Must be called from a crate PyO3 build script.
+/// Must be called from a PyO3 crate build script.
 fn is_linking_libpython_for_target(target: &Triple) -> bool {
     target.operating_system == OperatingSystem::Windows
         || target.environment == Environment::Android
@@ -1651,14 +1651,19 @@ pub fn make_cross_compile_config() -> Result<Option<InterpreterConfig>> {
 /// Only used by `pyo3-build-config` build script.
 #[allow(dead_code)]
 pub fn make_interpreter_config() -> Result<InterpreterConfig> {
+    let host = Triple::host();
     let abi3_version = get_abi3_version();
 
-    if have_python_interpreter() {
+    // See if we can skip the Python interpreter configuration detection.
+    // Unix "abi3" extension modules can be built without any interpreter,
+    // so do this by default just like maturin does.
+    let require_interpreter = abi3_version.is_none() || is_linking_libpython_for_target(&host);
+
+    if have_python_interpreter() && require_interpreter {
         let mut interpreter_config = InterpreterConfig::from_interpreter(find_interpreter()?)?;
         interpreter_config.fixup_for_abi3_version(abi3_version)?;
         Ok(interpreter_config)
     } else if let Some(version) = abi3_version {
-        let host = Triple::host();
         Ok(default_abi3_config(&host, version))
     } else {
         bail!("An abi3-py3* feature must be specified when compiling without a Python interpreter.")