gh-116869: Fix test_cext on RHEL7 (#117010)

Remove -std option from CC command line.

Skip C++14 test for now on non-Windows platforms (like RHEL7).

Lib/test/test_cext/setup.py

@@ -39,14 +39,17 @@ def main():
     if std:
         if support.MS_WINDOWS:
             cflags.append(f'/std:{std}')
-            std_prefix = '/std'
         else:
             cflags.append(f'-std={std}')
-            std_prefix = '-std'
 
-        # Remove existing -std options to only test ours
+    # Remove existing -std or /std options from CC command line.
+    # Python adds -std=c11 option.
     cmd = (sysconfig.get_config_var('CC') or '')
     if cmd is not None:
+        if support.MS_WINDOWS:
+            std_prefix = '/std'
+        else:
+            std_prefix = '-std'
         cmd = shlex.split(cmd)
         cmd = [arg for arg in cmd if not arg.startswith(std_prefix)]
         cmd = shlex.join(cmd)

Lib/test/test_cppext/__init__.py

@@ -35,6 +35,9 @@ class TestCPPExt(unittest.TestCase):
     def test_build_cpp11(self):
         self.check_build('_testcpp11ext', std='c++11')
 
+    # Only test C++14 on MSVC.
+    # On s390x RHEL7, GCC 4.8.5 doesn't support C++14.
+    @unittest.skipIf(not support.MS_WINDOWS, "need Windows")
     def test_build_cpp14(self):
         self.check_build('_testcpp14ext', std='c++14')
 

Lib/test/test_cppext/setup.py

@@ -35,14 +35,18 @@ def main():
     if std:
         if support.MS_WINDOWS:
             cppflags.append(f'/std:{std}')
-            std_prefix = '/std'
         else:
             cppflags.append(f'-std={std}')
-            std_prefix = '-std'
 
-        # Remove existing -std options to only test ours
+    # gh-105776: When "gcc -std=11" is used as the C++ compiler, -std=c11
+    # option emits a C++ compiler warning. Remove "-std11" option from the
+    # CC command.
     cmd = (sysconfig.get_config_var('CC') or '')
     if cmd is not None:
+        if support.MS_WINDOWS:
+            std_prefix = '/std'
+        else:
+            std_prefix = '-std'
         cmd = shlex.split(cmd)
         cmd = [arg for arg in cmd if not arg.startswith(std_prefix)]
         cmd = shlex.join(cmd)