I've been trying to build with ocamlbuild for several hours, and I keep getting the following error:

Error: No implementations provided for the following modules:

         Camlp4 referenced from /home/xyz/.opam/4.02.1/lib/comparelib/pa_compare.cmxa(Pa_compare),

           /home/xyz/.opam/4.02.1/lib/enumerate/enumerate.cmxa(Pa_enumerate),

           /home/xyz/.opam/4.02.1/lib/herelib/pa_herelib.cmxa(Pa_here),

           /home/xyz/.opam/4.02.1/lib/fieldslib/pa_fields_conv.cmxa(Pa_fields_conv),

           /home/xyz/.opam/4.02.1/lib/variantslib/pa_variants_conv.cmxa(Pa_variants_conv)

         Pa_type_conv referenced from /home/xyz/.opam/4.02.1/lib/comparelib/pa_compare.cmxa(Pa_compare),

           /home/xyz/.opam/4.02.1/lib/enumerate/enumerate.cmxa(Pa_enumerate),

           /home/xyz/.opam/4.02.1/lib/fieldslib/pa_fields_conv.cmxa(Pa_fields_conv),

           /home/xyz/.opam/4.02.1/lib/variantslib/pa_variants_conv.cmxa(Pa_variants_conv)

Clearly, this can be easily interpreted; it can't find camlp4 or the other. But I've tried all manner of -pkg camlp4 or -lib camlp4 (and similarly with Pa_type_conv, enumerate fieldslib and variantslib) and it's not picking these two up. I've checked and camlp4 is installed. I've looked in the ~/.opam directory and camlp4lib.cmxa and camlp4fulllib.cmxa are each there.