define PKG_CHECK_VAR if it doesn't exist
define PKG_CHECK_VAR if it doesn't exist

Fixes #19

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