CSTD sets -std= in CFLAGS, but not CXXFLAGS. It should do it for both (the reason why I didn't commit a change immediately is that it requires discussion and testing via a ports tinderbox).
Since the standard names for C and C++ are different, you cannot really use the CSTD variable. I would suggest using CXXSTD, which is consistent with all the other "CXX" prefixes.
The available options should probably be:
* c++14 (these are only available from clang 3.5 and higher)
Although the usefulness of the 03, 0x and 1y 'standards' is debatable, at least for the base system. For ports, all of these should obviously be available.