Fix Cygwin bootstrap error due to TM symbols

	PR libstdc++/69506
	* config/os/newlib/os_defines.h (_GLIBCXX_USE_WEAK_REF): Define.

From-SVN: r232974

libstdc++-v3/ChangeLog

+2016-01-29  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/69506
+	* config/os/newlib/os_defines.h (_GLIBCXX_USE_WEAK_REF): Define.
+
 2016-01-28  John David Anglin  <danglin@gcc.gnu.org>
 
 	PR libstdc++/69450

libstdc++-v3/config/os/newlib/os_defines.h

@@ -53,6 +53,9 @@
 // their dtors are called
 #define _GLIBCXX_THREAD_ATEXIT_WIN32 1
 
+// See libstdc++/69506
+#define _GLIBCXX_USE_WEAK_REF 0
+
 #endif
 
 #endif