Set cmake compile options based on language

[danimtb]

Changelog: Fix: Set CMake targets compile options based on language
Docs: omit

#tags: slow

• Refer to the issue that supports this Pull Request: closes [bug] Cmake targets glue C and CXX flags #7499
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
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[memsharded]

Looking good. Some tests that check generated CMake code are broken as expected.

[danimtb]

removing this variable when using components, as this feature is marked as experimental

[danimtb]

here I kept the variable (although it is not used in targets) to avoid breaking

[danimtb]

removed variable in the components flavour

[danimtb]

removed code not used

[danimtb]

I see the following errors in Windows:

Error evaluating generator expression:

    $<COMPILE_LANGUAGE:C>

  $<COMPILE_LANGUAGE:...> may not be used with Visual Studio generators.

I think we cannot implement it this way 😞

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See https://gitlab.kitware.com/cmake/cmake/-/issues/18582

[danimtb]

After trying to continue with this using the project language as described here https://stackoverflow.com/questions/32389273/detect-project-language-in-cmake, implementing this feels hacky to me.

First, we would need to extract the information of the language to a new variable with get_property(languages GLOBAL PROPERTY ENABLED_LANGUAGES) from the global scope.

Then, using a generator expression with IN_LIST requires CMake 3.3 as the minimum required version, while targets can be used since 3.1.2 version. However, I think you need to run CMake 3.12 at least to use it in generator expressions https://cmake.org/cmake/help/v3.12/manual/cmake-generator-expressions.7.html

To conclude, I am not sure if we should support this flag differentiation while creating the targets as CMake does not provide a feature supported in the main compilers. We can, however, expose the flags in variables without mixing them and let the consumers use that information in the way they want.