Try -std=gnu++14 before c++14 (gh-280)

aksetup_helper.py

@@ -917,7 +917,9 @@ def cpp_flag(compiler):
 
     The c++14 is prefered over c++11 (when it is available).
     """
-    if has_flag(compiler, '-std=c++14'):
+    if has_flag(compiler, '-std=gnu++14'):
+        return '-std=gnu++14'
+    elif has_flag(compiler, '-std=c++14'):
         return '-std=c++14'
     elif has_flag(compiler, '-std=c++11'):
         return '-std=c++11'