refactor: UNITS_UDLS renamed to UNITS_LITERALS

Also now the existence of the preprocessor flag is enough to trigger the feature (`#ifdef` instead of `#if`)

src/cmake/AddUnitsModule.cmake

@@ -45,7 +45,5 @@ endfunction()
 #
 function(add_units_system name)
     add_units_module(${name} ${ARGN})
-    if(UNITS_UDLS)
-        target_compile_definitions(mp-units-${name} INTERFACE UNITS_UDLS=1)
-    endif()
+    target_compile_definitions(mp-units-${name} INTERFACE $<$<BOOL:${UNITS_LITERALS}>:UNITS_LITERALS>)
 endfunction()