python: only link extension modules against libpython when building f…

…or Windows. Fixes mesonbuild#4117

Windows requires things to be linked, on macOS distutils doesn't link by default.
On Linux etc. things are not so clear, some distros patch distutils to not link,
some don't. In addition the manylinux wheels spec disallows linking against libpython.

Just assume for now that not linking on non-Windows works.

mesonbuild/modules/python.py

@@ -319,6 +319,21 @@ def extension_module(self, interpreter, state, args, kwargs):
 
             kwargs['install_dir'] = os.path.join(self.platlib_install_path, subdir)
 
+        # On macOS and some Linux distros distutils doesn't link extensions
+        # against libpython. We could call into distutils to mirror the default
+        # behavior, but let's keep it simple for now and only link on Windows.
+        # More details: https://github.com/mesonbuild/meson/issues/4117
+        env = interpreter.environment
+        link_libpython = mesonlib.for_windows(env.is_cross_build(), env) or mesonlib.for_cygwin(env.is_cross_build(), env)
+        if not link_libpython:
+            new_deps = []
+            for holder in kwargs.get('dependencies', []):
+                dep = holder.held_object
+                if isinstance(dep, PythonDependency):
+                    holder = interpreter.holderify(dep.get_partial_dependency(compile_args=True))
+                new_deps.append(holder)
+            kwargs['dependencies'] = new_deps
+
         suffix = self.variables.get('EXT_SUFFIX') or self.variables.get('SO') or self.variables.get('.so')
 
         # msys2's python3 has "-cpython-36m.dll", we have to be clever