Failed to Install camlp5 in Gentoo Linux - "make inconsistent assumptions over implementation Pretty"

When I use emerge to install camlp5-5.14 with ocaml-3.11.2 (or camlp5-5.12 with ocaml-3.10.2) and with ocamlopt enabled in Gentoo Linux, I get the following error message:

File "_none_", line 1, characters 0-1:
Error: Files prtools.cmx and ../lib/pretty.cmx
       make inconsistent assumptions over implementation Pretty

To solve this problem, I temporarily move /usr/lib/ocaml/pretty.cmx to some other place (or rename it). Don't forget to move the file back after camlp5 is installed successfully.

Added on 2010/11/29: I found the reason. The error message occurs because I installed the libraries of CIL in /usr/lib/ocaml by a self-written ebuild file. To prevent this error every time upgrading camlp5, I should install the CIL libraries in a separate folder.