PAM module unknown

[Robert Svensson]

Hi,
I can compile the FreeRADIUS PAM module without any errors. But when I try to use it I always get a module unknown error. For example:
AUTH-PAM: BACKGROUND: user 'john' failed to authenticate: Module is unknown

This is how I compiled the module:

gcc -fPIC -c pam_radius_auth.c
gcc -shared -o pam_radius_auth.so pam_radius_auth.o -lpam

What did I do wrong?