Index: lib/Basic/Targets/OSTargets.h
===================================================================
--- lib/Basic/Targets/OSTargets.h
+++ lib/Basic/Targets/OSTargets.h
@@ -505,6 +505,8 @@
 
     Builder.defineMacro("__rtems__");
     Builder.defineMacro("__ELF__");
+    if (Opts.CPlusPlus)
+      Builder.defineMacro("_GNU_SOURCE");
   }
 
 public:
Index: test/Preprocessor/init.c
===================================================================
--- test/Preprocessor/init.c
+++ test/Preprocessor/init.c
@@ -8955,6 +8955,7 @@
 // KFREEBSDI686-DEFINE:#define __GLIBC__ 1
 //
 // RUN: %clang_cc1 -x c++ -triple i686-pc-linux-gnu -fobjc-runtime=gcc -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix GNUSOURCE %s
+// RUN: %clang_cc1 -x c++ -triple sparc-rtems-elf -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix GNUSOURCE %s
 // GNUSOURCE:#define _GNU_SOURCE 1
 //
 // RUN: %clang_cc1 -x c++ -std=c++98 -fno-rtti -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix NORTTI %s