[patch] libstdc++/69506 Fix Cygwin bootstrap error due to TM symbols

[Jonathan Wakely]

Another target that doesn't have the necessary weak ref support for
the TM-aware exception-handling.

Bootrapped successfully by the reporter, committed to trunk.

commit b7e2e38ab1d938ee19280eba11ed3643a140f86d
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Fri Jan 29 10:38:45 2016 +0000

    Fix Cygwin bootstrap error due to TM symbols
    
    	PR libstdc++/69506
    	* config/os/newlib/os_defines.h (_GLIBCXX_USE_WEAK_REF): Define.

diff --git a/libstdc++-v3/config/os/newlib/os_defines.h b/libstdc++-v3/config/os/newlib/os_defines.h
index 4a09dd1..2a87e74 100644
--- a/libstdc++-v3/config/os/newlib/os_defines.h
+++ b/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