define AS_COPY_VAR in varnish.m4 if not already defined (autoconf < 2.63b)

found at: http://www.open-mpi.org/community/lists/devel/2010/09/8472.php

varnish.m4

@@ -28,6 +28,11 @@
 # SUCH DAMAGE.
 #
 
+# For compatibility with autoconf < 2.63b
+m4_ifndef([AS_VAR_COPY],
+  [m4_define([AS_VAR_COPY],
+     [AS_LITERAL_IF([$1[]$2], [$1=$$2], [eval $1=\$$2])])]) 
+
 # VARNISH_VMOD_INCLUDE_DIR([])
 # ----------------------------