Using std::copysign with g++

I just figured out some incompatibilities between my tested platforms using std::copysign.

At a first test I try in MSVC to get some algorithms tested. Now I tried to port this c++(11) code to the Xilinx Zynq platform (A9+FPGA).

This is working so far except there is no std::copysign including cmath. So I have to use copysign without the namespace.

What would be the proper way of adding this function to the namespace?

PS: I do not want to manage the same files on different platforms by hand just for namespace incompatibilities.

Thank you