build: configure failing on CentOS 6
$ ./configure
...
checking for reentrant lex... flex
checking for bison... bison -y
checking for systemd... no
./configure: line 13696: syntax error near unexpected token `fi'
./configure: line 13696: `fi'
https://jenkins.labs.nic.cz/view/Knot_DNS/job/Knot_DNS_master/Slaves=Centos-6.5-64b/1775/console