Makefile: fix targets run-server & run-client with GnuTLS 3.5.9
* Makefile (keydir/tls-privkey.pem keydir/tls-pubkey.pem): If the TLS session keys were not generated by mandos-keygen (which will happen with GnuTLS version 3.5.9 or older), create dummy empty unused files to avoid confusing make(1), which would otherwise re-run this target and create new OpenPGP key files which would not match the key fingerprint in the confdir/clients.conf file.
|| dpkg --compare-versions "$2" eq "1.8.0-1~bpo9+1"; then
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if grep --quiet --regexp='^[[:space:]]*key_id[[:space:]]*=[[:space:]]*[Ee]3[Bb]0[Cc]44298[Ff][Cc]1[Cc]149[Aa][Ff][Bb][Ff]4[Cc]8996[Ff][Bb]92427[Aa][Ee]41[Ee]4649[Bb]934[Cc][Aa]495991[Bb]7852[Bb]855[[:space:]]*$' /etc/mandos/clients.conf; then