postgres in 'C' - can't get started

Red Hat 7, Vanilla install postgres from RPM's

Create very simple program to open/close (cut down vsn of example 1)

Compile using libpq.a

Follwing error...

/usr/lib/libpq.a (fe-auth.a): In function `pg_password_sendauth`:
fe-auth.o(.text+0x30): undefined reference to `crypt`

New to postgres, have I missed something?  Reply, however terse, much appreciated.
Thanks
Tom K