build: use absolute path for cmarkTargets.cmake

[compnerd]

Adjust the include of the CMake file to use a cmarkConfig.cmake relative
location which enables use without considerations for the path.

[compnerd]

CC: @nwellnhof

[compnerd]

@nwellnhof - do you think it is better to just substitute in the absolute path instead from CMake?

[nwellnhof]

I have no idea.

[compnerd]

@jgm what do you think of merging this as is; once it is merged, I can update the version being used in Swift and we can revisit that if it is an issue.

[jgm]

I really don't understand the implications of this change. (Sorry, cmake is a mystery to me.) Can you explain a bit more fully?

[compnerd]

Oh, sure. The change is related to CMake's concepts of export targets - it allows for you to specify a set of targets which can then be used by another project. One way of using them is without installation of the library to the system. This allows you to build a static version of a library (e.g. cmark) and then directly link that into your program with proper dependency tracking across the projects. This mechanism works through a file that is emitted which is cmarkConfig.cmake (generated from cmarkConfig.cmake.in). This file references the cmarkTargets.cmake which is emitted by CMake as well. However, it was being included by a relative path and so unless you were in the correct location, it would fail to find the file. This now uses the absolute path which makes it resilient to the location from where things are being executed.

[jgm]

OK, sounds like a good idea then.

[drodriguez]

I think this change would have needed to modify the CMakeLists.txt invocation of configure_file to use the @ONLY parameter, because the CMAKE_CURRENT_LIST_DIR is being replaced during the configure_file, not during the include (according to the console logs of swiftlang/swift#29438).