Someone

gcc -shared -Wl,-soname,dict_ldap.so -o dict_ldap.so dict_ldap.o -lldap -llber ../../lib/libpostfix.so

% ldd -r dict_ldap.so
        linux-gate.so.1 =>  (0xffffe000)
        libldap-2.3.so.0 => /lib/libldap-2.3.so.0 (0xb7f3b000)
        liblber-2.3.so.0 => /lib/liblber-2.3.so.0 (0xb7f2f000)
        libpostfix-2.3.6.so => /usr/lib/libpostfix-2.3.6.so (0xb7ee3000)
        libc.so.6 => /lib/libc.so.6 (0xb7dc4000)
        libresolv.so.2 => /lib/libresolv.so.2 (0xb7db1000)
        libsasl2.so.2 => /lib/libsasl2.so.2 (0xb7d9b000)
        libssl.so.4 => /lib/libssl.so.4 (0xb7d6b000)
        libcrypto.so.4 => /lib/libcrypto.so.4 (0xb7c6f000)
        libnsl.so.1 => /lib/libnsl.so.1 (0xb7c5a000)
        /lib/ld-linux.so.2 (0x80000000)
        libdl.so.2 => /lib/libdl.so.2 (0xb7c55000)
        libcrypt.so.1 => /lib/libcrypt.so.1 (0xb7c22000)
undefined symbol: db_common_check_domain        (./dict_ldap.so)
undefined symbol: db_common_parse       (./dict_ldap.so)
undefined symbol: db_common_expand      (./dict_ldap.so)
undefined symbol: db_common_parse_domain        (./dict_ldap.so)
undefined symbol: db_common_dict_partial        (./dict_ldap.so)
undefined symbol: db_common_free_ctx    (./dict_ldap.so)

% nm -D libpostfix-2.3.6.so |grep db_common_check_domain
0001308b T db_common_check_domain