cmake: Respect --prefix for headers during cmake --install

[ericvw]

Removing CMAKE_INSTALL_PREFIX allows for cmake --install <build-dir> --prefix=<installation-prefix> to install headers at <installation-prefix>/include at install time.

When CMAKE_INSTALL_PREFIX is present in INSTALL(), it is hard-coded during configuration (e.g., cmake -S .), which disallows it from being set at install time.

Closes: #782

[codecov-commenter]

Codecov Report

Merging #1060 (5ce1f4d) into c_master (8160ede) will not change coverage. The diff coverage is n/a.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## c_master #1060 +/- ## ========================================= Coverage 55.45% 55.45% ========================================= Files 8 8 Lines 1044 1044 ========================================= Hits 579 579 Misses 465 465 ```

[redboltz]

It seems that the CI (Linux) doesn't start. I think that it is caused by ubuntu18.04 end of support. And your PR is the first one after that.

I guess that the following line should be updated to ubuntu-latest or ubuntu-20.04. Could you update it in your PR ? https://github.com/msgpack/msgpack-c/blob/8b57d2caf98003154c4873c7887347669435ca9f/.github/workflows/gha.yml#L69

[ericvw]

I guess that the following line should be updated to ubuntu-latest or ubuntu-20.04. Could you update it in your PR ?

Sure. Which one do you prefer?

[ericvw]

I went with ubuntu-latest to be aligned with macos-latest.

[ericvw]

There is more work involved in updating Linux CI. I will work on that in a separate PR as it is becoming more involved.

[ericvw]

I created #1061 to get Linux CI updated…

[ericvw]

@redboltz, I rebased this PR against the latest c_master branch.

[redboltz]

Thank you! merged.