Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       erf():
           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
           or cc -std=c99
       erff(), erfl():
           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
           or cc -std=c99