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/cxxopts.cmake

@@ -91,7 +91,7 @@ function(cxxopts_install_logic)
         string(REPLACE "/${CMAKE_LIBRARY_ARCHITECTURE}" "" CMAKE_INSTALL_LIBDIR_ARCHIND "${CMAKE_INSTALL_LIBDIR}")
     else()
         # On some systems (e.g. NixOS), `CMAKE_LIBRARY_ARCHITECTURE` can be empty
-        set(CMAKE_INSTALL_LIBDIR_ARCHIND "${CMAKE_INSTALL_LIBDIR}")
+        set(CMAKE_INSTALL_LIBDIR_ARCHIND "${CMAKE_INSTALL_DATAROOTDIR}")
     endif()
     set(CXXOPTS_CMAKE_DIR "${CMAKE_INSTALL_LIBDIR_ARCHIND}/cmake/cxxopts" CACHE STRING "Installation directory for cmake files, relative to ${CMAKE_INSTALL_PREFIX}.")
     set(version_config "${PROJECT_BINARY_DIR}/cxxopts-config-version.cmake")