Don't force -std=c89, as this can conflict with HAVE_LONG_LONG from Python.h

see
https://mail.python.org/pipermail/cython-devel/2016-April/004776.html

Also note better support for gcc variants that are not named 'gcc' exactly.

runtests.py

@@ -858,8 +858,8 @@ class CythonCompileTestCase(unittest.TestCase):
             ext_compile_flags = CFLAGS[:]
             compiler = COMPILER or sysconfig.get_config_var('CC')
 
-            if self.language == 'c' and compiler == 'gcc':
-                ext_compile_flags.extend(['-std=c89', '-pedantic'])
+            if self.language == 'c' and 'gcc' in compiler:
+                ext_compile_flags.append('-pedantic')
             if  build_extension.compiler == 'mingw32':
                 ext_compile_flags.append('-Wno-format')
             if extra_extension_args is None: