kdc does not compile with glibc 2.8

kdc does not compile with glibc 2.8

On Fedora 9, glibc 2.8 is used.  The kdc code conditionalizes
IPV6_PKTINFO and HAVE_STRUCT_IN6_PKTINFO in a number of places = but
misses two for the struct one.

/usr/include/netinet/in.h conditionalizes struct in6_pktinfo on
__USE_GNU - which I believe implies a gnu libc extension.  People on
the net have defined GNU_SOURCE for various things to compile, etc.

I do note that /usr/include/linux/ipv6.h exists with the same definition.

I believe that ipv6 support in the kdc will not work with these changes - but
the tree compiles.

ticket: new

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@20498 dc483132-0cff-0310-8789-dd5450dbe970