smoketests: add a test for the limited API

This builds the test module twice, once normally and once for the
limited API. In the later case we check at runtime if libpython3.dll
is loaded, to make sure we are actually linked against it and not
against libpython3.x.dll.

This depends on both:
* #148
* msys2/MINGW-packages#18232

mingw_smoketests.py

@@ -265,6 +265,12 @@ def test_site(self):
         self.assertEqual(len(site.getsitepackages()), 1)
 
     def test_c_ext_build(self):
+        self._test_c_ext_build(False)
+
+    def test_c_ext_build_limited_api(self):
+        self._test_c_ext_build(True)
+
+    def _test_c_ext_build(self, py_limited_api):
         import tempfile
         import sys
         import subprocess
@@ -279,32 +285,47 @@ def test_c_ext_build(self):
                         """\
                                     from setuptools import setup, Extension
 
+                                    if %(py_limited_api)s:
+                                        ext = Extension(
+                                            'cwrapper',
+                                            py_limited_api=True,
+                                            define_macros=[('Py_LIMITED_API', '0x03060000')],
+                                            sources=['cwrapper.c'])
+                                    else:
+                                        ext = Extension(
+                                            'cwrapper',
+                                            sources=['cwrapper.c'])
+
                                     setup(
                                         name='cwrapper',
                                         version='1.0',
-                                        ext_modules=[
-                                            Extension(
-                                                'cwrapper',
-                                                sources=['cwrapper.c']),
-                                        ],
+                                        ext_modules=[ext],
                                     )
-                                """
+                                """ % {"py_limited_api": py_limited_api}
                     )
                 )
+
             with Path(tmppro, "cwrapper.c").open("w") as f:
                 f.write(
                     textwrap.dedent(
                         """\
                                     #include <Python.h>
+                                    #include <windows.h>
                                     static PyObject *
                                     helloworld(PyObject *self, PyObject *args)
                                     {
                                         printf("Hello World\\n");
                                         Py_RETURN_NONE;
                                     }
+                                    static PyObject *
+                                    islimited(PyObject *self, PyObject *args)
+                                    {
+                                        return PyBool_FromLong(GetModuleHandleA("libpython3.dll") != NULL);
+                                    }
                                     static PyMethodDef
                                     myMethods[] = {
                                         { "helloworld", helloworld, METH_NOARGS, "Prints Hello World" },
+                                        { "islimited", islimited, METH_NOARGS, "Returns True if the limited API is used" },
                                         { NULL, NULL, 0, NULL }
                                     };
                                     static struct PyModuleDef cwrapper = {
@@ -323,6 +344,16 @@ def test_c_ext_build(self):
                                 """
                     )
                 )
+
+            if py_limited_api:
+                with Path(tmppro, "setup.cfg").open("w") as f:
+                    f.write(textwrap.dedent(
+                        """\
+                        [bdist_wheel]
+                        py_limited_api=cp36
+                        """
+                    ))
+
             subprocess.check_call(
                 [sys.executable, "-c", "import struct"],
             )
@@ -347,6 +378,10 @@ def test_c_ext_build(self):
             subprocess.check_call(
                 [sys.executable, "-c", "import cwrapper"],
             )
+            # Make sure the resulting extension uses the limited API, or not
+            subprocess.check_call(
+                [sys.executable, "-c", f"import cwrapper; assert cwrapper.islimited() == {py_limited_api}"],
+            )