don't break install paths when DESTDIR is set

--- a/install.pl
+++ b/install.pl
@@ -131,7 +131,7 @@
 
 if( $destdir ) {
   for( $bindir, $datadir, $mandir, $htmldir_val ) {
-    s/$prefix/$destdir/o if defined;
+    s/^/$destdir/o if defined;
   }
 }