update ftk building script.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1077 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/external/ftk/SConscript

 # for module compiling
 import os
+from building import *
 Import('RTT_ROOT')
 
-objs = SConscript('ftk/src/os/rt-thread/SConscript')
+if os.path.isfile(os.path.join(RTT_ROOT, 'components', 'external', 'ftk',
+    'ftk/src/os/rt-thread/SConscript')):
+    objs = SConscript('ftk/src/os/rt-thread/SConscript')
+else:
+    if GetDepend('RT_USING_FTK'):
+        print '================ERROR============================'
+        print 'Please get ftk file and put them under ftk folder'
+        print '================================================='
+        exit(0)
+
+    objs = []
 
 Return('objs')