Update to rra-c-util 4.8

Update to rra-c-util 4.8

* Fix Heimdal libroken probes for old versions of Heimdal.
* Fix Kerberos header probing with non-standard include paths.
* Pass --deps to krb5-config if it is supported.
* Properly find krb5.h on NetBSD systems.
* Fix stripping of -I/usr/include from krb5-config output.
* Avoid using krb5-config if specific Kerberos paths are configured.
* Use PATH_KRB5_CONFIG instead of KRB5_CONFIG to locate krb5-config.
* Replace concat with xasprintf.
* xasprintf is now void and always calls the failure handler on error.
* Improve __attribute__ portability to old GCC or non-GCC compilers.
* Add -D_FORTIFY_SOURCE=2 to make warnings flags.
* Probe for ssize_t and replace it in portable/system.h if not found.
* Include strings.h in portable/system.h if it exists.
* Add a pointer to rra-c-util in all files.