capture C compiler output in test runner and discard it when expecting C compiler failures

runtests.py

@@ -867,15 +867,33 @@ class CythonCompileTestCase(unittest.TestCase):
 
         so_path = None
         if not self.cython_only:
+            c_compiler_output = None
+            orig_stderr = os.dup(2)  # remember original stderr
+            try:
+                with tempfile.TemporaryFile() as temp_stderr:
+                    os.dup2(temp_stderr.fileno(), 2)  # replace stderr by temp file
+                    try:
                         try:
                             so_path = self.run_distutils(test_directory, module, workdir, incdir)
+                        finally:
+                            temp_stderr.seek(0)
+                            c_compiler_output = temp_stderr.read().strip()
                     except Exception:
-                if expected_errors != '_FAIL_C_COMPILE':
+                        if expected_errors == '_FAIL_C_COMPILE':
+                            assert c_compiler_output
+                        else:
                             raise
                     else:
+                        if c_compiler_output:
+                            print("\n=== C/C++ compiler output: ===")
+                            print(c_compiler_output)
                         if expected_errors == '_FAIL_C_COMPILE':
                             # must raise this outside the try block
                             raise RuntimeError('should have failed C compile')
+                    finally:
+                        os.dup2(orig_stderr, 2)  # restore stderr
+            finally:
+                os.close(orig_stderr)
         return so_path
 
 class CythonRunTestCase(CythonCompileTestCase):