CMake: Don't add uninstall target and CPack config if not top-level

[ZedThree]

Fixes #2597

CMake 3.21 was released in 2021 and is available in the latest release of all the big distros (see https://pkgs.org/search/?q=cmake). If it's necessary to support older versions of CMake, a reasonable backport is something like:

if (${CMAKE_PROJECT_NAME} STREQUAL ${PROJECT_NAME})
  include(CMakeInstallation.cmake)
endif()

CMAKE_PROJECT_NAME is the top-level project name
PROJECT_NAME is the current project name (i.e. netCDF!)

It might also be friendlier to expose an option to allow changing this:

if (${CMAKE_PROJECT_NAME} STREQUAL ${PROJECT_NAME})
  set(_default_install_option ON)
else()
  set(_default_install_option OFF)
endif()

option(INSTALL_NETCDF "Install netCDF-C" ${_default_install_option})

if (INSTALL_NETCDF)
  include(CMakeInstallation.cmake)
endif()

[WardF]

Thanks! Re-running CI to get an idea of what didn't work, and hopefully see that it will clear itself up.

[ZedThree]

Oh, looks like the Windows build only has Cmake 3.16, so maybe it is best to use the backwards compatible version