Restore non-arch dependant path for the pkgconfig file (#1012)

Originally setup at #635 but
reverted in #916 without any
specific reason that I can find.

Author: j-rivero

cmake/CLI11GeneratePkgConfig.cmake

@@ -4,4 +4,4 @@ else()
   configure_file("cmake/CLI11.pc.in" "CLI11.pc" @ONLY)
 endif()
 
-install(FILES "${PROJECT_BINARY_DIR}/CLI11.pc" DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+install(FILES "${PROJECT_BINARY_DIR}/CLI11.pc" DESTINATION "${CMAKE_INSTALL_DATADIR}/pkgconfig")