ppx_optcomp_old.Make it easier to share a .h and .mlh
Ppx_optcomp was modified to accept defined(X) only if X has been seen, either in a #define or #undef. This allow to share config.h files between C and OCaml and still be protected against typos in .ml files.
-ppChange the way optcomp resolve filenames in #import directives
Do the same as cpp, i.e. for relative filenames, consider they are relative to the directory of the file being parsed. This doesn't matter internally as build commands are always executed from the current directory, but it matters for the public release as everything is executed from the root.