Update CMakeFile for ohPipeline

[DoomHammer]

CMAKE_BUILD_TYPE is apparently not set in Windows builds. The new approach should handle this.

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• I've read the contributing guidelines.
• I've used a recent Conan client version close to the currently deployed.
• I've tried at least one configuration locally with the conan-center hook activated.

[DoomHammer]

BTW: if there is a better way to handle the ServiceGen path handover than this: https://github.com/conan-io/conan-center-index/blob/master/recipes/ohnet/all/conanfile.py#L113 I'd be happy to learn. Somehow this feels extremely hacky.

[Croydon]

Needing to maintain a completely custom build system for a project is already a nightmare, but even more so when externalizing the scripts. I am not sure why this was accepted.

If it is too big to be included in CCI's repository, it is too big to be maintained. Can we please integrate it back here or have a general discussion about this?

[DoomHammer]

I've also created a separate PR that tries to work with existing build system but it fails in ways I can't reproduce :(

[memsharded]

Agree this should be done, but I know it can be complicated too.
So approving this fix in the meantime, thanks very much for contributing it @DoomHammer !

[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 5 (285a7bf8e8f596bb59a4c9b241d3344748eb66ff):

• ohpipeline/1.139.1000:
  All packages built successfully! (All logs)

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Conan v2 pipeline ✔️

Note: Conan v2 builds are now mandatory. Please read our discussion about it.

All green in build 5 (285a7bf8e8f596bb59a4c9b241d3344748eb66ff):

• ohpipeline/1.139.1000:
  All packages built successfully! (All logs)