As a package manager for a Linux distribution, I want to install docs into a separate prefix. With CMake projects, the docs installation location is controlled by CMAKE_INSTALL_DOCDIR from GNUInstallDirs module. Unfortunately, unlike the other directory variables, this one contains the project name so I cannot just use cmake "-DCMAKE_INSTALL_DOCDIR=$myDocPrefix/doc".
With GNU Make, I would run make "DOCDIR=$myDocPrefix/doc/\$(PROJECT_NAME)" and have Make interpolate it but the documentation of CMake’s -D option does not mention interpolation and I understand that CMake uses much more complex system of cache entries where interpolation might be problematic (especially if the referenced variable is not yet in cache).
I could pass tailor-made CMAKE_INSTALL_DOCDIR to each CMake project but would be bothersome as I would have to do that in every package definition manually; being able to define configureCmakeProject function and have it take care of everything automatically would be better. When setting it manually, I would also want to make sure it matches the PROJECT_NAME of the respective CMake project – well, I could resign on that and just use $packageName from the package definition instead but keeping packages as close to upstream as possible is preferred.
Alternately, I could try to grep CMakeLists.txt for project command but that seems fragile and might still result in misalignments. I doubt it is possible to extract it using some CMake API since the project is not configured at the time and we actually need the value to configure the project.
Is there a way I can configure CMAKE_INSTALL_DOCDIR to use custom prefix but still keep the project name set by the CMake project?