cmake: make pre-built library discoverable via CMake configuration files (rebased to master)

[jeongseok-meta]

Hello, I'm creating this PR to help move forward with PR 7 in case the only blocker is that it hasn't been rebased onto the master branch. Please feel free to merge this PR or update the original PR by simply rebasing it.

Closing #7

[wjakob]

Sorry about the huge delay in looking at this PR. Is it still relevant? I am wondering if we can avoid adding a CMake-internal config file to the root directory (nanothreadConfig.cmake). It seems to me that this can just be auto-generated.

[jeongseok-meta]

Yes, it'd be much cleaner. Updated!

[wjakob]

Isn't it redundant to use file(WRITE and configure_package_config_file? AFAIK the second is just a way of substituting CMake variables, but we can already do that when using file(WRITE

[jeongseok-meta]

AFAIK, we need two steps: first, generate a file that contains @PACKAGE_INIT@ (via file(WRITE ...)), and then generate the final config file that expands @PACKAGE_INIT@ (via configure_package_config_file(...)). This is necessary because configure_package_config_file(...) only accepts a file as input.

[jeongseok-meta]

In any case, this is just one idea to help move the original PR forward, so please feel free to adjust it as needed :)