[external] fix the VC++ simulation issue for lodepng.

components/external/lodepng/SConscript

@@ -1,4 +1,5 @@
 from building import *
+import rtconfig
 
 cwd = GetCurrentDir()
 src = Split('''
@@ -7,6 +8,9 @@ lodepng.c
 
 CPPPATH = [cwd]
 
-group = DefineGroup('RTGUI', src, depend = ['RT_USING_RTGUI', 'RTGUI_IMAGE_LODEPNG', 'RT_USING_LIBC'], CPPPATH = CPPPATH)
+if rtconfig.ARCH == 'sim':
+	group = DefineGroup('RTGUI', src, depend = ['RT_USING_RTGUI', 'RTGUI_IMAGE_LODEPNG'], CPPPATH = CPPPATH)
+else:
+	group = DefineGroup('RTGUI', src, depend = ['RT_USING_RTGUI', 'RTGUI_IMAGE_LODEPNG', 'RT_USING_LIBC'], CPPPATH = CPPPATH)
 
 Return('group')