krb5-config (used by FindGSSAPI) returns `-lkrb5 -lk5crypto -lcom_err`
but only libkrb5 is actually used by the gssapi plugin. The other two
result in unneeded dependencies unless they're tagged optional when
linked with --as-needed.

Some distributions use --as-needed automatically, which causes our
builds to differ from srpm builds, introducing failures in buildbot.

Let's always use --as-needed for gssapi plugin