Building higher fails on bytecode only switches

[dbuenzli]

/cc @pqwy

The package should not try to compile native code if opam's ocaml-native variable is false. Using oasis or topkg may solve the problem.

# opam-version 1.2.2
# os           darwin
# command      make
# path         /Users/dbuenzli/.opam/4.03.0+effects/build/higher.0.2
# compiler     4.03.0+effects
# exit-code    2
# env-file     /Users/dbuenzli/.opam/4.03.0+effects/build/higher.0.2/higher-58668-1872ec.env
# stdout-file  /Users/dbuenzli/.opam/4.03.0+effects/build/higher.0.2/higher-58668-1872ec.out
# stderr-file  /Users/dbuenzli/.opam/4.03.0+effects/build/higher.0.2/higher-58668-1872ec.err
### stdout ###
# ocamldep higher.mli > ._bcdi/higher.di
# ocamldep higher.ml > ._d/higher.d
# ocamlc -c higher.mli
# ocamlc -c higher.ml
# ocamlc -a                -o higher.cma higher.cmo
# ocamldep -native higher.mli > ._ncdi/higher.di
# ocamlopt -c higher.ml
### stderr ###
# File "higher.ml", line 1:
# Error: higher.cmi
# is not a compiled interface for this version of OCaml.
# It seems to be for a newer version of OCaml.
# make[1]: *** [higher.cmx] Error 2
# make: *** [native-code-library] Error 2