Custom firmware include c++ (2011)

I am trying to include my own code in the espruino firmware. My code is partly written in c++. But if I try to build it, it failed:

Espruino/xtensa-esp32-elf/xtensa-esp32-elf/include/c++/5.2.0/bits/c++0x_warning.h:32:2: error: error This file requires compiler and library support for the ISO C++ 2011 standard. This support is currently experimental, and must be enabled with the -std=c++11 or -std=gnu++11 compiler options.

I tried several ways to set the -std=c++ flag in the Makefile, but I couldn't resolve it.

(If I mixed something up, I apologize, C is not my native language 😅)

Jonas