undefined reference to `derf'

I compile others' codes ,I find it have a function below, I compile .......

Double Precision Function myderf(a)
Implicit None
Double Precision a
Double Precision derf
External derf
myderf=derf(a)

Return

End Function myderf

I got errors

ze_i386.o: In function `myderf_':
ze_i386.f90:(.text+0x1b): undefined reference to `derf'
ze_i386.o: In function `myderfc_':
ze_i386.f90:(.text+0x3b): undefined reference to `derfc'

why ?  need  I  link something ?