Keep --c99 flag and append MDK5 building notification.

Author: wosayttn

bsp/nuvoton/numaker-m2354/README.md

@@ -39,7 +39,9 @@ Support GCC, MDK5, IAR IDE/compilers. More information of these compiler version
 | IAR        | 8.32                                 |
 | GCC        | GCC 6.3.1 20170620 (Need update env) |
 
-Notice: Please install Nu-Link_Keil_Driver or Nu-Link_IAR_Driver for development.
+Notice: 
+(1) Please install Nu-Link_Keil_Driver or Nu-Link_IAR_Driver for development.
+(2) You must remove '--c99' flag in <path/to/rtt>/components/drivers/spi/SConscript before generating MDK5 project file.
 
 ## 3. Program firmware
 ### Step 1:

components/drivers/spi/SConscript

@@ -28,7 +28,7 @@ if GetDepend('RT_USING_SFUD'):
     if rtconfig.CROSS_TOOL == 'gcc':
         LOCAL_CCFLAGS += ' -std=c99'
     elif rtconfig.CROSS_TOOL == 'keil':
-        LOCAL_CCFLAGS += ' '
+        LOCAL_CCFLAGS += ' --c99'
 
 src += src_device