Fix a number of packaging issues

[dcbaker]

This little series fixes or improves a number of package issues:

• the pkg-config file has an empty Description: field
• the pkg-config file now has it's URL field set
• On NixOS, using CMAKE_INSTALL_DATAROOTDIR will put files in the proper arch-independent location
• the pkg-config file now has it's Requires field and Cflags correctly configured when ICU is used
• the CMake files and pkg-config files are properly installed in arch-specific locations when ICU is used. Since ICU is a arch specific the packaging files are also arch specific when used with ICU

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This change is 

[jarro2783]

Thanks, looks good to me.