fixed newlib building script error.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1748 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/libc/newlib/SConscript

 Import('rtconfig')
-
 from building import *
 
-if rtconfig.CROSS_TOOL != 'gcc':
+if GetDepend('RT_USING_NEWLIB') and rtconfig.CROSS_TOOL != 'gcc':
     print '================ERROR============================'
     print 'Please use GNU GCC compiler if using newlib'
     print '================================================='