unit tests: Run native file tests on the CI

The native file tests were never run on the CI since they were skipped
on Windows and also skipped on Linux and macOS since CC/CXX/etc are
always set by the CI.

Also fix test failure on macOS. The test was assuming that because
/usr/bin/gcc and /usr/bin/clang exist on macOS, they must be different
compilers. They're not. gcc is just a wrapper around clang, and we
correctly detect it as such.

run_unittests.py

@@ -104,6 +104,16 @@ def _git_init(project_dir):
     subprocess.check_call(['git', 'commit', '-a', '-m', 'I am a project'], cwd=project_dir,
                           stdout=subprocess.DEVNULL)
 
+@functools.lru_cache()
+def is_real_gnu_compiler(path):
+    '''
+    Check if the gcc we have is a real gcc and not a macOS wrapper around clang
+    '''
+    if not path:
+        return False
+    out = subprocess.check_output([path, '--version'], universal_newlines=True, stderr=subprocess.STDOUT)
+    return 'Free Software Foundation' in out
+
 def skipIfNoExecutable(exename):
     '''
     Skip this test if the given executable is not found.
@@ -165,14 +175,23 @@ def skip_if_not_language(lang):
         return wrapped
     return wrapper
 
-def skip_if_env_value(value):
+def skip_if_env_set(key):
+    '''
+    Skip a test if a particular env is set, except when running under CI
+    '''
     def wrapper(func):
         @functools.wraps(func)
         def wrapped(*args, **kwargs):
-            if value in os.environ:
-                raise unittest.SkipTest(
-                    'Environment variable "{}" set, skipping.'.format(value))
-            return func(*args, **kwargs)
+            old = None
+            if key in os.environ:
+                if not is_ci():
+                    raise unittest.SkipTest('Env var {!r} set, skipping'.format(key))
+                old = os.environ.pop(key)
+            try:
+                return func(*args, **kwargs)
+            finally:
+                if old is not None:
+                    os.environ[key] = old
         return wrapped
     return wrapper
 
@@ -5217,59 +5236,59 @@ class NativeFileTests(BasePlatformTests):
         self._simple_test('python', 'python')
 
     @unittest.skipIf(is_windows(), 'Setting up multiple compilers on windows is hard')
-    @skip_if_env_value('CC')
+    @skip_if_env_set('CC')
     def test_c_compiler(self):
         def cb(comp):
             if comp.id == 'gcc':
                 if not shutil.which('clang'):
                     raise unittest.SkipTest('Only one compiler found, cannot test.')
                 return 'clang', 'clang'
-            if not shutil.which('gcc'):
+            if not is_real_gnu_compiler(shutil.which('gcc')):
                 raise unittest.SkipTest('Only one compiler found, cannot test.')
             return 'gcc', 'gcc'
         self.helper_for_compiler('c', cb)
 
     @unittest.skipIf(is_windows(), 'Setting up multiple compilers on windows is hard')
-    @skip_if_env_value('CXX')
+    @skip_if_env_set('CXX')
     def test_cpp_compiler(self):
         def cb(comp):
             if comp.id == 'gcc':
                 if not shutil.which('clang++'):
                     raise unittest.SkipTest('Only one compiler found, cannot test.')
                 return 'clang++', 'clang'
-            if not shutil.which('g++'):
+            if not is_real_gnu_compiler(shutil.which('g++')):
                 raise unittest.SkipTest('Only one compiler found, cannot test.')
             return 'g++', 'gcc'
         self.helper_for_compiler('cpp', cb)
 
     @skip_if_not_language('objc')
-    @skip_if_env_value('OBJC')
+    @skip_if_env_set('OBJC')
     def test_objc_compiler(self):
         def cb(comp):
             if comp.id == 'gcc':
                 if not shutil.which('clang'):
                     raise unittest.SkipTest('Only one compiler found, cannot test.')
                 return 'clang', 'clang'
-            if not shutil.which('gcc'):
+            if not is_real_gnu_compiler(shutil.which('gcc')):
                 raise unittest.SkipTest('Only one compiler found, cannot test.')
             return 'gcc', 'gcc'
         self.helper_for_compiler('objc', cb)
 
     @skip_if_not_language('objcpp')
-    @skip_if_env_value('OBJCXX')
+    @skip_if_env_set('OBJCXX')
     def test_objcpp_compiler(self):
         def cb(comp):
             if comp.id == 'gcc':
                 if not shutil.which('clang++'):
                     raise unittest.SkipTest('Only one compiler found, cannot test.')
                 return 'clang++', 'clang'
-            if not shutil.which('g++'):
+            if not is_real_gnu_compiler(shutil.which('g++')):
                 raise unittest.SkipTest('Only one compiler found, cannot test.')
             return 'g++', 'gcc'
         self.helper_for_compiler('objcpp', cb)
 
     @skip_if_not_language('d')
-    @skip_if_env_value('DC')
+    @skip_if_env_set('DC')
     def test_d_compiler(self):
         def cb(comp):
             if comp.id == 'dmd':
@@ -5285,7 +5304,7 @@ class NativeFileTests(BasePlatformTests):
         self.helper_for_compiler('d', cb)
 
     @skip_if_not_language('cs')
-    @skip_if_env_value('CSC')
+    @skip_if_env_set('CSC')
     def test_cs_compiler(self):
         def cb(comp):
             if comp.id == 'csc':
@@ -5298,7 +5317,7 @@ class NativeFileTests(BasePlatformTests):
         self.helper_for_compiler('cs', cb)
 
     @skip_if_not_language('fortran')
-    @skip_if_env_value('FC')
+    @skip_if_env_set('FC')
     def test_fortran_compiler(self):
         def cb(comp):
             if comp.id == 'gcc':
@@ -5331,13 +5350,13 @@ class NativeFileTests(BasePlatformTests):
         self.assertEqual(compiler.version, version)
 
     @skip_if_not_language('vala')
-    @skip_if_env_value('VALAC')
+    @skip_if_env_set('VALAC')
     def test_vala_compiler(self):
         self._single_implementation_compiler(
             'vala', 'valac', 'Vala 1.2345', '1.2345')
 
     @skip_if_not_language('rust')
-    @skip_if_env_value('RUSTC')
+    @skip_if_env_set('RUSTC')
     def test_rust_compiler(self):
         self._single_implementation_compiler(
             'rust', 'rustc', 'rustc 1.2345', '1.2345')