CMake: Activate ccache

[ladislas]

Summary of changes

This PR activates ccache for CMake builds as discussed here in #13477

#13477 (comment)

Impact of changes

The changes should speed up the build time by using ccache.

Migration actions required

n/a

Documentation

n/a

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@0xc0170 @hugueskamba

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[ciarmcom]

@ladislas, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

Shouldn't this be

find_program(CCACHE ccache)
if(CCACHE)
    set(CMAKE_C_COMPILER_LAUNCHER ${CCACHE})
    set(CMAKE_CXX_COMPILER_LAUNCHER ${CCACHE})
endif(CCACHE)

I found this https://reviews.bitcoinabc.org/D5150?id=15995.

This PR is fine as it is but the other approach looks more future proof.

[ladislas]

@0xc0170 I'm happy to change but I'm still not sure which of the two methods is considered best practice.

llvm use my approach here: https://github.com/llvm/llvm-project/blob/905f874c449cc114d74eaeb19639664779fd0b6e/llvm/CMakeLists.txt#L153

Same as opencv: https://github.com/opencv/opencv/blob/master/cmake/OpenCVCompilerOptions.cmake#L20

The CMake documentation about RULE_LAUNCH_COMPILE is also interesting: https://cmake.org/cmake/help/latest/prop_gbl/RULE_LAUNCH_COMPILE.html

This is intended to allow launchers to intercept build problems with high granularity.

Regarding your reference and their issue with clang-tidy, it seems that CMAKE_<LANG>_CLANG_TIDY would be a better fit for them, but I've never tried it:
https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_CLANG_TIDY.html

Maybe someone with deep CMake knowledge can shine in on that :)

[0xc0170]

Yes, both are fine. I would go with updated version as clang-tidy is something we might revisit in the future so it works in both cases (with and without clang-tidy or something else).

[0xc0170]

We update the testing, when you update, please also rebase.

[ladislas]

I'll do the change, rebase, rename the branch and make a new PR.