Went with:

@@ -1835,7 +1835,8 @@ ATF_TC_BODY(isc_crc64, tc) {
                                       testcase->input_len);
                }
                isc_crc64_final(&crc);
-               tohexstr((unsigned char *) &crc, sizeof(crc), str, sizeof(str));
+               snprintf(str, sizeof(str),
+                        "0x%016" ISC_PRINT_QUADFORMAT "X", crc);
                ATF_CHECK_STREQ(str, testcase->result);

and appropriate changes to the expected output strings.

-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: marka@isc.org