fast_float_math_hack.h works only when __ICL is defined (that is, when FLAC is compiled with Intel compiler). But current Intel compilers don't have mathf.h file. Instead, mathimf.h should be used, and mathimf.h is not compatible with the standard math.h. Maybe it's time to remove fast_float_math_hack.h? It is useful only for those who use ICC compiled FLAC and use --apply-replaygain-which-is-not-lossless switch with hard -6dB limiting.