Fix cmake rules to support higher-level targest (#3377) (#3386).

Don't add linker flags as generator expression to support linking `TARGET` to higher-level targets in a top-level CMakeLists.txt file.

docs/CONTRIBUTORS

@@ -71,6 +71,7 @@ Markus Krause
 Marlon James
 Marshal Qiao
 Martin Schmidt
+Martin Stadler
 Matthew Ballance
 Michael Killough
 Michaël Lefebvre

verilator-config.cmake.in

@@ -334,9 +334,14 @@ function(verilate TARGET)
 
   target_link_libraries(${TARGET} PUBLIC
     ${${VERILATE_PREFIX}_USER_LDLIBS}
-    "$<$<BOOL:$<TARGET_PROPERTY:VERILATOR_THREADED>>:${VERILATOR_MT_CFLAGS}>"
   )
 
+  if (${VERILATE_PREFIX}_THREADS OR ${VERILATE_PREFIX}_TRACE_THREADS)
+    target_link_libraries(${TARGET} PUBLIC
+      ${VERILATOR_MT_CFLAGS}
+    )
+  endif()
+
   target_compile_features(${TARGET} PRIVATE cxx_std_11)
 endfunction()