Prevent the literal string DEBUG from being appended to some messages in CMake < 3.15 (#433)

Fixes issue #422
diff --git a/pico_sdk_init.cmake b/pico_sdk_init.cmake
index c291a50..b3e054c 100644
--- a/pico_sdk_init.cmake
+++ b/pico_sdk_init.cmake
@@ -18,6 +18,15 @@
         endif()
     endfunction()
 
+    function(pico_message_debug MESSAGE)
+        # The log-level system was added in CMake 3.15.
+        if(${CMAKE_VERSION} VERSION_LESS "3.15.0")
+            message(${MESSAGE})
+        else()
+            message(DEBUG ${MESSAGE})
+        endif()
+    endfunction()
+
     if (NOT PICO_SDK_PATH)
         set(PICO_SDK_PATH ${CMAKE_CURRENT_LIST_DIR})
     endif ()
@@ -49,14 +58,14 @@
     macro(add_sub_list_dirs var)
         foreach(LIST_DIR IN LISTS ${var})
             get_filename_component(SHORT_NAME "${LIST_DIR}" NAME)
-            message(DEBUG "Including custom CMakeLists.txt ${SHORT_NAME}")
+            pico_message_debug("Including custom CMakeLists.txt ${SHORT_NAME}")
             add_subdirectory(${LIST_DIR} ${SHORT_NAME})
         endforeach()
     endmacro()
 
     macro(add_sub_list_files var)
         foreach(LIST_FILE IN LISTS ${var})
-            message(DEBUG "Including custom CMake file ${LIST_FILE}")
+            pico_message_debug("Including custom CMake file ${LIST_FILE}")
             include(${LIST_FILE})
         endforeach()
     endmacro()