What version of OR-Tools and what language are you using?
Version: Latest (git clone of stable branch)
Language: Attempting to build Python bindings

Which solver are you using (e.g. CP-SAT, Routing Solver, GLOP, BOP, Gurobi)
N/A

What operating system (Linux, Windows, ...) and version?
OpenBSD-current

What did you do?
Steps to reproduce the behavior:

1. $ git clone --depth=1 https://github.com/google/or-tools.git
2. # pkg_add lzlib abseil-cpp protobuf eigen3 re2
3. Build COIN-OR dependencies:

$ for dep in CoinUtils Osi Clp Cgl Cbc
do git clone --depth=1 https://github.com/coin-or/$dep $dep
done

$ for dep in CoinUtils Osi Clp Cgi
do cd $dep ; ./configure -C --prefix=/home/myuser/coin --exec-prefix=/home/myuser/coin ;
make && make install
clean
cd ..
done

The Cbc repository includes a compilation error on BSD systems that must be corrected. Replace the reinterpret_cast at src/CbcModel.cpp line ~6255 with static_cast. This is due to the way NULL is defined on BSD systems. See this commit for more information.

Then:

$ ./configure -C --prefix=/home/myuser/coin --exec-prefix=/home/myuser/coin ;
make && make install
clean

4. In your installation virtual environment, pip install pybind11.
5. Determine the path to pybind11's cmake files (this can be done easily by starting python with the verbose flag and importing pybind11).
6. Add the following line to or-tools/CMakeLists.txt to point to pybind11's CMake files:

set(pybind11_DIR /home/myuser/venv/lib/python3.11/site-packages/pybind11/share/cmake/pybind11)
find_package(pybind11 REQUIRED)

7. Locate pybind11_protobuf project here.
8. Search and search and search for installation instructions or a guide on pointing the or-tools CMake configuration to a cloned copy of pybind11_protobuf
9. Configure LD_LIBRARY_PATH to include the installed COIN-OR libraries as well as any BLAS on your system (I use libopenblas):

        export LD_LIBRARY_PATH=/home/myuser/coin/lib:/home/myuser/libopenblas/lib

11. Repeatedly try turning flags on and off while building or-tools:

        cmake -S. -Bbuild \
            -DBUILD_PYTHON=ON \
            -DUSE_SCIP=OFF \
            -DBUILD_DEPS=OFF

What did you expect to see

A successful build producing a python package suitable for use on platforms for which no "wheel" is prebuilt on pypi.org.

What did you see instead?

CMake Error at cmake/deps.cmake:169 (find_package):
  By not providing "Findpybind11_protobuf.cmake" in CMAKE_MODULE_PATH this
  project has asked CMake to find a package configuration file provided by
  "pybind11_protobuf", but CMake did not find one.

  Could not find a package configuration file provided by "pybind11_protobuf"
  with any of the following names:

    pybind11_protobufConfig.cmake
    pybind11_protobuf-config.cmake

  Add the installation prefix of "pybind11_protobuf" to CMAKE_PREFIX_PATH or
  set "pybind11_protobuf_DIR" to a directory containing one of the above
  files.  If "pybind11_protobuf" provides a separate development package or
  SDK, be sure it has been installed.
Call Stack (most recent call first):
  CMakeLists.txt:435 (include)

Anything else we should know about your project / environment

For operating systems other than macOS, GNU/Linux, and some Windows flavors, there are no prebuilt wheels for the python or-tools library. This means pip install or-tools fails to find a candidate:

$ pip install or-tools
ERROR: Could not find a version that satisfies the requirement or-tools (from versions: none)
ERROR: No matching distribution found for or-tools

As such, one must resort to building the library from source. Luckily several dependencies are prebuilt for my platform (see the pkg_add step above) and the COIN-OR projects are easier to build than their documentation portrays. My current roadblock is this pybind11_protobuf piece. It is admittedly outside my area of expertise, so without documentation detailing installation or integration procedures, I am left with a guess-test-and-revise strategy.