#2144: Fix unwanted cache export

[thearusable]

Fixes #2144

[github-actions]

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PR tests (clang-9, ubuntu, mpich)

Build for 801cbc4 (2023-05-31 15:51:55 UTC)

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PR tests (clang-12, ubuntu, mpich)

Build for 801cbc4 (2023-05-31 15:51:55 UTC)

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PR tests (gcc-10, ubuntu, openmpi, no LB)

Build for 801cbc4 (2023-05-31 15:51:55 UTC)

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PR tests (gcc-9, ubuntu, mpich, zoltan, json schema test)

Build for 801cbc4 (2023-05-31 15:51:55 UTC)

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PR tests (clang-11, ubuntu, mpich)

Build for 801cbc4 (2023-05-31 15:51:55 UTC)

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PR tests (clang-13, ubuntu, mpich)

Build for 801cbc4 (2023-05-31 15:51:55 UTC)

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PR tests (clang-14, ubuntu, mpich)

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PR tests (clang-10, ubuntu, mpich)

Build for 801cbc4 (2023-05-31 15:51:55 UTC)

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PR tests (gcc-11, ubuntu, mpich, trace runtime, coverage)

Build for 801cbc4 (2023-05-31 15:51:55 UTC)

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PR tests (gcc-8, ubuntu, mpich, address sanitizer)

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PR tests (gcc-12, ubuntu, mpich)

Build for 801cbc4 (2023-05-31 15:51:55 UTC)

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PR tests (intel icpc, ubuntu, mpich)

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PR tests (nvidia cuda 11.2, gcc-9, ubuntu, mpich)

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[nlslatt]

@thearusable What is the new behavior introduced by this PR? Does it avoid forcing ccache on the parent project but still use it for vt embedded in that other project's build?

[PhilMiller]

@thearusable What is the new behavior introduced by this PR? Does it avoid forcing ccache on the parent project but still use it for vt embedded in that other project's build?

I realize reading this PR that I should have thought a bit more about what to say in opening this and DARMA-tasking/magistrate#302. It's one thing if vt or Magistrate uses ccache automatically when built on its own. When built as part of a larger project, it should build the same way the larger project does, and not try to customize things like that.

[thearusable]

@nlslatt Yes. After the latest changes it will respect the CMAKE_CXX_COMPILER_LAUNCHER parameter. It will search for ccache only when it's not defined. In other cases it will respect whatever was set in CMAKE_CXX_COMPILER_LAUNCHER for this or parent project.

[thearusable]

After update vt will only search for ccache in standalone mode when CMAKE_CXX_COMPILER_LAUNCHER is not defined.

[nlslatt]

Looks great