INSTALL: add --enable-relocatable-bundle in options list.

(cherry picked from commit c543103783)

INSTALL.in

@@ -217,6 +217,10 @@ These are:
   --disable-vector-icons.  This option installs raster icons instead of
      vector icons.
 
+  --enable-relocatable-bundle.  This option forces GIMP to search some
+     resources (e.g. MyPaint brushes or libwmf fonts) relatively to the
+     running prefix, rather than using build-time paths.
+
   --enable-shared and --disable-shared.  This option affects whether
      shared libraries will be built or not. Shared libraries provide
      for much smaller executables. The default is to enable shared