Support for OCaml 4.08

[xclerc]

There are actually few changes impacting ppx_include:

• error reporting;
• access to the load path.

[whitequark]

#=== ERROR while installing ppx_include.1.1 ===================================#
# opam-version 1.2.2
# os           linux
# command      ocaml pkg/build.ml native=true native-dynlink=true
# path         /home/travis/.opam/system/build/ppx_include.1.1
# compiler     system (4.02.3)
# exit-code    10
# env-file     /home/travis/.opam/system/build/ppx_include.1.1/ppx_include-9899-6d07ae.env
# stdout-file  /home/travis/.opam/system/build/ppx_include.1.1/ppx_include-9899-6d07ae.out
# stderr-file  /home/travis/.opam/system/build/ppx_include.1.1/ppx_include-9899-6d07ae.err
### stdout ###
# ocamlfind ocamlopt -package unix -package ocamlbuild -linkpkg -package cppo_ocamlbuild myocamlbuild.ml /usr/lib/ocaml/ocamlbuild/ocamlbuild.cmx -o myocamlbuild
# + ocamlfind ocamlopt -package unix -package ocamlbuild -linkpkg -package cppo_ocamlbuild myocamlbuild.ml /usr/lib/ocaml/ocamlbuild/ocamlbuild.cmx -o myocamlbuild
# ocamlfind: Package `cppo_ocamlbuild' not found
# Command exited with code 2.