[libc] Add THREAD_SUPPORT and FILE_DESCRIPTOR in dlib.

components/libc/dlib/SConscript

@@ -9,6 +9,10 @@ CPPPATH = [cwd]
 CPPDEFINES = ['RT_USING_DLIBC']
 
 if rtconfig.PLATFORM == 'iar':
+
+    if GetDepend('RT_USING_DFS'):
+        CPPDEFINES = CPPDEFINES + ['_DLIB_FILE_DESCRIPTOR', '_DLIB_THREAD_SUPPORT']
+
     group = DefineGroup('dlib', src, depend = ['RT_USING_LIBC'], 
         CPPPATH = CPPPATH, CPPDEFINES = CPPDEFINES)