Fix GCC 4.x compilation error

Author: onelife

library.properties

@@ -1,5 +1,5 @@
 name=RT-Thread
-version=0.9.3
+version=0.9.4
 author=onelife <onelife.real@gmail.com>, Bernard Xiong <bernard.xiong@gmail.com>
 maintainer=onelife <onelife.real@gmail.com>
 sentence=Real Time Operating System porting for Arduino SAM and SAMD boards

src/rtconfig.h

@@ -500,8 +500,10 @@
 
 /* User provided config */
 
-#if __has_include("rtconfig_extra.h")
-# include "rtconfig_extra.h"
+#if defined __has_include
+# if __has_include("rtconfig_extra.h")
+#  include "rtconfig_extra.h"
+# endif
 #endif
 
 #endif /* __RTCONFIG_H__ */

src/shell_cmd.h

@@ -109,8 +109,10 @@ ADD_MSH_CMD(prtscn, capture screen, prtscn, int, int argc, char **argv)
 # endif
 #endif
 
-#if __has_include("user_cmd.h")
-# include "user_cmd.h"
+#if defined __has_include
+# if __has_include("user_cmd.h")
+#  include "user_cmd.h"
+# endif
 #endif