fftwQuad: Fix build on Darwin by forcing gcc

  fftwSinglePrec = fftw.override { precision = "single"; };

  fftwFloat = fftwSinglePrec; # the configure option is just an alias

  fftwLongDouble = fftw.override { precision = "long-double"; };

  fftwQuad = fftw.override { precision = "quad-precision"; };

  # Need gcc >= 4.6.0 to build with FFTW with quad precision, but Darwin defaults to Clang

  fftwQuad = fftw.override {

    precision = "quad-precision";

    stdenv = gccStdenv;

  };

  fftwMpi = fftw.override { enableMpi = true; };

  filter-audio = callPackage ../development/libraries/filter-audio {};