* cmake: set PROJECT_DESCRIPTION and PROJECT_HOMEPAGE_URL after project()
Otherwise they are set to an empty string.
* cmake: set the pkg-config URL field
Since the information is already there to set it.
* cmake: use CMAKE_INSTALL_DATAROOTDIR if CMAKE_LIBRARY_ARCHITECTURE is unset
This causes files on NixOS to be put in the proper architecture
independent place, which otherwise was selecting the architecture
dependent location.
* cmake: Properly set pkg-config requires when configured with ICU
In this case the pkg-config file needs to set icu-cu in the `Required`
field, and needs to add the flag `-DCXXOPTS_USE_UNICODE` to the `Cflags`
field.
* cmake: cxxopts is not arch independent when built with ICU support
Since it links to an architecture dependent ICU
