Avoid duplicate "/etc/krb5.conf" in profile path

Avoid duplicate "/etc/krb5.conf" in profile path

If configure gets run with --sysconfdir=/etc, "/etc/krb5.conf" shows
up twice in the profile path, which causes its contents to be read
twice.  This can cause some confusing and possibly problematic
behavior.

Add some logic to configure.in to avoid adding the duplicate entry for
"/etc/krb5.conf".

Reported independently by Denis Vlasenko and Fredrik Tolf.

(back ported from commit 8df1965d1ccdbcb8d74a3e68ad35d7579db9ac44)

ticket: 3277
version_fixed: 1.12.2
status: resolved