Fix macOS compilation, set c++14 as highest

setup.py

@@ -70,7 +70,7 @@ def cpp_flag(compiler):
 
     The newer version is prefered over c++11 (when it is available).
     """
-    flags = ['-std=c++17', '-std=c++14', '-std=c++11']
+    flags = ['-std=c++14', '-std=c++11']
 
     for flag in flags:
         if has_flag(compiler, flag): return flag