Require Varnish 6.0.0.

Varnish 5.2.x has the bug with redefinition of _Static_assert, we
won't mess around with it.

configure.ac

@@ -30,7 +30,7 @@ AC_ARG_WITH([rst2man],
 
 m4_ifndef([VARNISH_PREREQ], AC_MSG_ERROR([Need varnish.m4 -- see INSTALL.rst]))
 
-VARNISH_PREREQ([5.2.0])
+VARNISH_PREREQ([6.0.0])
 VARNISH_VMODS([dispatch])
 
 VMOD_TESTS="$(cd $srcdir/src && echo tests/*.vtc)"