Fix CMake package config file

[gmgunter]

There are currently a few issues with how the COIN-OR dependencies are resolved when using find_package(ortools CONFIG) to add or-tools to an external CMake project.

• With CMake >=3.9.6, we try to find the Clp and Cbc packages using the CONFIG mode of find_package. This always fails since neither coin-or/Clp nor coin-or/Cbc provides a CMake config file.

• If we address the above issue by instead using the MODULE mode of find_package, then we need to add FindCbc.cmake and FindCpl.cmake scripts to the CMAKE_MODULE_PATH in order to teach CMake how to find these dependencies. I propose that or-tools should install these scripts alongside its config file so that they're available to external CMake projects.

• Finally, the FindCbc.cmake script included with or-tools defines the variable CBC_FOUND when it's successful, rather than Cbc_FOUND as expected (CMake variables are case-sensitive). The FindCpl.cmake script has a similar issue. As a result, even when the above two points are addressed, find_package(ortools CONFIG) will still fail because CMake erroneously thinks that these two dependencies weren't succesfully found.

This PR attempts to address the above issues with the following changes:

• The CMake config file is modified to allow searching for Cbc and Cpl using the MODULE mode of find_package.

• FindCbc.cmake and FindCpl.cmake are installed with the CMake package config files and are added to the CMAKE_MODULE_PATH if USE_COINOR was truthy.

• The FindCbc.cmake and FindCpl.cmake files are modified to change the case of the variables they export.

[Mizux]

1. I've forked coin-or repos (Cbc, Clp, Cgl, Osi, CoinUtils) few years ago to add CMake support, they never want to integrate it...
   see: Add CMake-based build support coin-or/CoinUtils#101
   exhaustive list: Add CMake-based build support coin-or/CoinUtils#101 (comment)

   1. Currently we only test/use with options USE_COINOR=ON and BUILD_DEPS=ON aka using coin-or through
      FetchContent()

2. vcpkg already contains a CMake port for Clp but it's not complete...
   src: https://github.com/microsoft/vcpkg/tree/master/ports/clp

3. Good catch for CBC/Cbc and CLP/Clp need to double check pkg_check_modules

   1. Need a job to test the CMake based build of or-tools against coin-or installed system-wide
      e.g. using an ubuntu 20.04 LTS image
      ref: https://repology.org/project/coin-or-cbc/versions

[gmgunter]

I've forked coin-or repos (Cbc, Clp, Cgl, Osi, CoinUtils) few years ago to add CMake support, they never want to integrate it...

That's too bad. The most elegant solution would definitely be for these upstream packages to add CMake support.

Need a job to test the CMake based build of or-tools against coin-or installed system-wide

This would be really nice. I installed or-tools via the conda-forge package, which builds with BUILD_DEPS=OFF and relies on finding independently installed packages for all the dependencies.

[Mizux]

FYI I'll need to change the module CMake since:

1. It does not support super build i.e. if Cbc target already present the module should be a noop
2. It does not support cbc_config.cmake file (like we have when building cbc or using vcpkg) so first module should try a to perform a find_package( CONFIG) then if found -> nothing to do, otherwise try to locate it using the pkg plugin...

On my way to fix it, if you have some regression in your own workflow, please let me know...

[gmgunter]

Sounds good 👌

[Neumann-A]

@Mizux I'll probably be the one to rip out all the CMakeLists.txt vcpkg vendors. Including any exported targets which have been added and are currently in disagreement with the maintainer guidelines of vcpkg. Either you get upstream to accept your CMake versions or pkg-config will be the preferred way to consume coin-or libraries.

[Mizux]

@Neumann-A

1. Do you have a link to this maintainer guidelines assertion ?
2. Does pkg-config build will work on Windows and nicely integrate with vcpkg ?
3. How to deal (nicely) with a CMake toolchain file if using pkg-config/autotools based build ?
4. How about having a Community supported CMake fork ?

note: here pkg-config/autotools based build means what coin-or provide if I understand correctly your point
note2: IIRC it was a user of openframework who provide a first ugly CMakeLists.txt patch on vcpkg

[Neumann-A]

Do you have a link to this maintainer guidelines assertion ?

a) The current name of most coin-or ports in vcpkg is wrong according to https://github.com/microsoft/vcpkg/blob/master/docs/maintainers/maintainer-guide.md#check-names-against-other-repositories (basically the ports need a prefix coin-or-*

b) unofficial:: namespace: https://github.com/microsoft/vcpkg/blob/master/docs/maintainers/maintainer-guide.md#add-cmake-exports-in-an-unofficial--namespace

Does pkg-config build will work on Windows and nicely integrate with vcpkg ?
How to deal (nicely) with a CMake toolchain file if using pkg-config/autotools based build ?

vcpkg has a port for pkgconf. If it is installed CMake'sFindPkgConfig will automatically start working. be aware that it breaks multi-config usage (e.g. no interactive switching of the build type, as long as you stay with one build type you are fine). For ports internal to vcpkg it will start to automatically work due to how the vcpkg_ function are setup and if additional downstream usage agrees with upstream provided usage.

How about having a Community supported CMake fork ?

No. Upstream it or leave it. Source packages have to come from the authoritative source.

So I already started ripping it out: microsoft/vcpkg#29398. basically the only thing left to do is fixing openmvg and getting the linux VM to have autoconf 2.71