libstdc++: Darwin does not support weak refs without definition.

        PR libstdc++/69310
        * config/os/bsd/darwin/os_defines.h (_GLIBCXX_USE_WEAK_REF): Define.

From-SVN: r232628

libstdc++-v3/ChangeLog

+2016-01-20  Torvald Riegel  <triegel@redhat.com>
+
+	PR libstdc++/69310
+	* config/os/bsd/darwin/os_defines.h (_GLIBCXX_USE_WEAK_REF): Define.
+
 2016-01-20  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/60401

libstdc++-v3/config/os/bsd/darwin/os_defines.h

@@ -47,4 +47,7 @@
 #define _GLIBCXX_USE_NANOSLEEP 1
 #define _GLIBCXX_USE_SCHED_YIELD 1
 
+// No support for referencing weak symbols without a definition.
+#define _GLIBCXX_USE_WEAK_REF 0
+
 #endif