Compiling with libc++ issues

[brad-goss]

Libc++ requires the std::polar function to input a non negative rho.

https://cplusplus.github.io/LWG/issue2459

iOS12 is removing libstdc++ from the simulator runtime and is pushing devs to adopt libc++.

I am able to build DSPFilters, by setting the DSPFilters build settings "C++ Standard Library" to "Compiler Default" and removing any build references to libstdc++.

While the library now compiles and runs, it hits an assertion.

Changing Butterworth.cpp lines 90/91, removing the negative sign seems to resolve the issue.