Is it possible to specify a compiler flag through preprocessor directives?

Question

C++ / C#

Preview:
I'm participating in a programming competition. Wrote working code in C++14. But on the verification server, GCC 5.1 by default runs code without the -std=c++14 flag. Specifying a flag from the client side on the site is not possible. There is no desire to rewrite the code in C++03.
The question itself is:
How to specify a compiler flag through preprocessor directives, like #pragma ?
Googled, didn't find anything