Revise config.cmake generation

[LecrisUT]

CMake already provides built-in support for making sure the package is relocatable. This PR contains:

• Use CMakePackageConfigHelpers helpers
• Changed INSTALL_CMAKE_DIR from share to CMAKE_INSTALL_LIBDIR. share is only supposed to be used for architecture independent packages. Changed it for Windows as well because it is generally a common practice to use GNUInstallDirs compatible paths for Windows as well (see vcpkg), but feel free to alter it if you have different needs there.
• Made the @PROJECT_NAME@_*_DIRS calculated from PATH_VARS.
  • Note it might break if CMAKE_INSTALL_* vars is not relative to CMAKE_INSTALL_PREFIX, but the previous approach is also prone to breaking
  • PS: please find a way to deprecate these, e.g. move them to a DEPRECATED component and emit a warning if these are used
• Changed INSTALL_NAME_DIR to use genex to be more relocatable
• Changed the standard to more modern naming conventions -config.cmake -> Config.cmake, -depends.cmake -> Targets.cmake. Please see about changing EXPORT depends as well

[hobu]

libgeotiff's CMake configuration is very old. I think this activity is good alignment with modern CMake, but we need to be careful to let the packagers who might have been scaffolding around our crufty configuration catch up on the next release.

[LecrisUT]

I come from Fedora packaging ¹ and this is part of the removal of questionable default options in cmake macro. But for sure try to contact the other distro packagers. Generally it should be valid out-of-the-box because GNUInstallDirs accounts for each distribution standards.

Footnotes

1. https://src.fedoraproject.org/rpms/libgeotiff/pull-request/2 ↩