Update to rra-c-util 6.3 and C TAP Harness 3.4

Update to rra-c-util 6.3 and C TAP Harness 3.4

Update to rra-c-util 6.3:

* Fix new warnings in GCC 7.
* Probe for warning flags instead of hard-coding a list.
* New test for obsolete URLs and email addresses.
* Remove unused portable replacements for strlcpy and strlcat.
* Use C_TAP_SOURCE and C_TAP_BUILD environment variables in tests.
* Fix portability defines for anonymous principal strings.
* Clear errno on pam_modutil_getpwnam to improve other testing.
* Add portability defines for macOS's PAM implementation.
* Add new Autoconf macro to probe for pam_strerror const usage.
* Support Solaris 10's included Kerberos.

Update to C TAP Harness 3.4:

* Fix segfault in runtests with an empty test list.
* Display verbose test results with -v or C_TAP_VERBOSE.
* Test infrastructure builds cleanly with Clang warnings.