RedHat: Do not force krb5-config path

RedHat: Do not force krb5-config path

If the %krb5config macro is not defined, do not force using
/usr/kerbers/bin/krb5-config, since sometimes that is not where it is
(RHEL6 puts it in /usr/bin). Instead only specify KRB5_CONFIG if
krb5config is defined; otherwise let configure find krb5-config for
us.

Change-Id: I35cc03ddbba7edd2c03d8f72b9636d4a7f6e64c0
Reviewed-on: http://gerrit.openafs.org/2642
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>