Add cmake warning if multiple SOURCES w/o PREFIX (#3916) (#3927)

verilator-config.cmake.in

@@ -142,6 +142,10 @@ function(verilate TARGET)
   endif()
 
   if (NOT VERILATE_PREFIX)
+    list(LENGTH VERILATE_SOURCES NUM_SOURCES)
+    if (${NUM_SOURCES} GREATER 1)
+      message(WARNING "Specify PREFIX if there are multiple SOURCES")
+    endif()
     list(GET VERILATE_SOURCES 0 TOPSRC)
     get_filename_component(_SRC_NAME ${TOPSRC} NAME_WE)
     set(VERILATE_PREFIX V${_SRC_NAME})