MFC r311140:

Only bake krb5_config.h support in to ssh(3), etc if both MK_GSSAPI and
MK_KERBEROS_SUPPORT != no

This fixes the odd case where someone specified MK_GSSAPI=no and
MK_KERBEROS_SUPPORT=yes (which admittedly, probably doesn't make sense,
but the build system doesn't prevent this case today, and it didn't when
I filed the bug back in 2011 either).

PR: 159745