Adding a basic CMakeLists for building a static library

[Coeur]

Based on @ckerr code at https://github.com/transmission/libpsl/blob/692c69d2415e1b20378030c08607935a54434087/CMakeLists.txt

Adapted for upstream.

Added a cross-platforms workflow to trigger it on each pull request (ubuntu, windows, macos).

[rockdaboot]

Please add CMakeLists.txt to EXTRA_DIST in Makefile.am. That adds the file to the tarball, so that the instructions also work when building from tarball.

[eli-schwartz]

Asking for the sake of clarity...

What's the overall desired goal for CMake support. e.g. is this about using it via add_subdirectory from another CMake project?

[Coeur]

In my use case (Transmission), yes:

1. We have libpsl as submodule under "third-party".
2. We do add_subdirectory("third-party/libpsl" "${PROJECT_BINARY_DIR}/third-party/libpsl.bld")

https://github.com/transmission/transmission/blob/b372f7b193a2c02864af33093295d26b7caacfb7/CMakeLists.txt#L554-L555
https://github.com/transmission/transmission/blob/b372f7b193a2c02864af33093295d26b7caacfb7/cmake/TrMacros.cmake#L148

[eli-schwartz]

How do you stop Transmission from installing the static library and header file of its bundled dependency and possibly conflicting with a separately installed libpsl system package?

[Coeur]

@ckerr @mikedld @tearfur can you help me here?
I'm trying to include libpsl's CMakeLists upstream, but I do not know how to answer that question.

Should we remove that line?

[mikedld]

We don't need to prevent the installation; on the contrary, it's very much expected to happen for file to end up in a predictable location. The library is used via ExternalProject module though, so the installation is not happening as part of Transmission installation, but as a step of an isolated build with install prefix pointing somewhere inside the build tree: https://github.com/transmission/transmission/blob/0f7f460c5589c5511c18140c321df5f85c0b7233/cmake/TrMacros.cmake#L201

That said, if we were to include libpsl as a subproject then definitly having an option like e.g. LIBPSL_ENABLE_INSTALL (defaulting to OFF for subproject builds) would be useful. See https://github.com/transmission/libb64/blob/post-2.0.0-transmission/CMakeLists.txt for an example.

[eli-schwartz]

Thanks for clarifying. For ExternalProject it makes sense to automatically install the results -- less so for add_subdirectory (or FetchContent).

The distinguishing factor is that ExternalProject doesn't actually expect to install libpsl as part of Transmission, it is just doing so into a custom temporary directory as part of recursive (multistage) builds.

(e.g. this approach is also generally incompatible with defining C macros whose value relates to CMAKE_INSTALL_PREFIX)

[Coeur]

OK, here is what I came up with:
40d0f64