inherit the default prefix from Varnish and document behaviour

configure.ac

@@ -53,6 +53,7 @@ AS_VAR_IF([$1], [""], [$5], [$4])dnl
 ])
 
 PKG_CHECK_MODULES([libvarnishapi], [varnishapi])
+PKG_CHECK_VAR([LIBVARNISHAPI_PREFIX], [varnishapi], [prefix])
 PKG_CHECK_VAR([LIBVARNISHAPI_DATAROOTDIR], [varnishapi], [datarootdir])
 PKG_CHECK_VAR([LIBVARNISHAPI_BINDIR], [varnishapi], [bindir])
 PKG_CHECK_VAR([LIBVARNISHAPI_SBINDIR], [varnishapi], [sbindir])
@@ -62,6 +63,9 @@ AC_SUBST([LIBVARNISHAPI_DATAROOTDIR])
 VARNISH_VMOD_INCLUDES
 VARNISH_VMOD_DIR
 VARNISH_VMODTOOL
+# AC_PREFIX_DEFAULT acts too early in the configure run,
+# so we can't use it
+ac_default_prefix=$LIBVARNISHAPI_PREFIX
 
 AC_PATH_PROG([VARNISHTEST], [varnishtest], [],
     [$LIBVARNISHAPI_BINDIR:$LIBVARNISHAPI_SBINDIR:$PATH])