Get dune build working on Windows

[jonahbeckford]

During capnp-ocaml development I want dune build to work. That compiles the whole project, including the benchmark code. dune build makes it easy to see if I'm breaking other code, even if I can't run the benchmarks (yet) on Windows.

Two changes:

• CAPNP_INCLUDE environment variable can be set if the build machine does not have a standard location to store capnp/*.capnp schema file (ex. /usr/include on Unix)
• Windows compiler flags and commands are detected and used in benchmarks

I also added in manual compile instructions for Windows.

The python benchmark tool was not modified. Running the benchmark on Unix is fine for now.

[jonahbeckford]

I'm a bit confused about why CAPNP_INCLUDE is needed. If we can find the capnp binary OK on Windows, can't that can find the includes relative to itself?

capnp is a binary not under our control. Are you asking for changes to the upstream capnp binary? Because that mechanism does not exist today as far as I can tell: https://capnproto.org/language.html#imports has the rules for the search path.

EDIT: Here is the capnp search path logic: https://github.com/capnproto/capnproto/blob/6e071e34d88a8fc489638535899cd9d02e55bf76/c%2B%2B/src/capnp/compiler/capnp.c%2B%2B#L350-L367.

[talex5]

Yes, it seems reasonable to get this in the upstream binary. Otherwise, we'll have to do this dune stuff in every project that uses capnp.

[jonahbeckford]

Yes, it seems reasonable to get this in the upstream binary. Otherwise, we'll have to do this dune stuff in every project that uses capnp.

If I were the maintainer of capnp I would say no to that request. I would question why the requester's build system and/or the package system does not support functions, aliases, macros ... anything so that this logic could be centralized and not repeated. I would also question why the requester wasn't using the build system (CMake) that capnp uses, which supports all of that. I would also do a quick google search and see that there is already a proposal in Dune to make it much more friendly to the CMake and has a specific example for capnp: https://github.com/ocaml/dune/discussions/8707#capnp-code-generation.

I'll let you ask for the upstream issue. In the meantime, we can keep this issue open. I'll update the other two minor and uncontroversial things you mentioned (-O3 and arg passing) when I get some free time.