Repair incorrect return value usage in test runner when checking gcc version....

Repair incorrect return value usage in test runner when checking gcc version. Fails on Windows when gcc is not available (with an error trying to regex parse None).

runtests.py

@@ -269,7 +269,7 @@ def update_cpp11_extension(ext):
         update cpp11 extensions that will run on versions of gcc >4.8
     """
     gcc_version = get_gcc_version(ext.language)
-    if gcc_version is not None:
+    if gcc_version:
         compiler_version = gcc_version.group(1)
         if float(compiler_version) > 4.8:
             ext.extra_compile_args.append("-std=c++11")
@@ -295,7 +295,7 @@ def get_cc_version(language):
        cc = ccompiler.get_default_compiler()
 
     if not cc:
-        return None
+        return ''
 
     # For some reason, cc can be e.g. 'gcc -pthread'
     cc = cc.split()[0]
@@ -309,7 +309,7 @@ def get_cc_version(language):
         # Be compatible with Python 3
         warnings.warn("Unable to find the %s compiler: %s: %s" %
                       (language, os.strerror(sys.exc_info()[1].errno), cc))
-        return None
+        return ''
     _, output = p.communicate()
     return output.decode(locale.getpreferredencoding() or 'ASCII', 'replace')