The C code didn't have -Werror enabled (when configured) and didn't
enable the full set of warnings we would want.
However OCaml-C bindings commonly declare functions which are
called directly from OCaml, eg:
external parse_memory : string -> doc = "v2v_xml_parse_memory"
value v2v_xml_parse_memory (value xmlv)
These do not require prototypes as no other C code will call them, so
we have to switch off the GCC -Werror=missing-prototypes flag for
these files.
Also fixes some confusion between _CPPFLAGS and _CFLAGS.