Support configuring on EL6.
Support configuring on EL6.

mbgrydeland/libvmod-fsbackend,aondio/libvmod-cowsay,varnish/libvmod-example,Dridi/libvmod-example,varnish/libvmod-example,aondio/libvmod-rtfrontend,aondio/libvmod-rtfrontend,Dridi/libvmod-example,aondio/libvmod-rtfrontend,mbgrydeland/libvmod-fsbackend,aondio/libvmod-cowsay