It is expected that CMake packages can be found multiple times without
errors. The target generated and defined by CMake, e.g. in a
*Targets.cmake file have a similar guard.
Howard Hinnant's date and time library is found using a cmake Find-module.
Commit 377fbc8958b26cb2c0151b3068bce07dff3811ec was reverted to ensure that the
exported target has the correct properties.
