remove c++ standard from compile options and define separately to compile with clang

[mlanighan]

When building with clang I ran into an issue with the specified compile options.
gcc throws a warning for this, but clang throws an error.

building with gcc:
cc1: warning: command line option ‘-std=c++14’ is valid for C++/ObjC++ but not for C

building with clang:
error: invalid argument '-std=c++14' not allowed with 'C'

By defining the c++ standard outside of compile options this warning/error is addressed and I can compile with both clang and gcc.