allow selection of default cache compressor

http://bugs.gentoo.org/205147

--- a/configure
+++ b/configure
@@ -1012,6 +1012,10 @@
       done
     done
 
+    if [ x$COMPRESS != x ]; then
+      compress=$COMPRESS
+    fi
+
     if [ x$default = x ]; then
       echo ""
       echo "What program should be used to compress the formatted pages?"