Index: abi/include/_bits/decls.h
===================================================================
--- abi/include/_bits/decls.h	(revision 2b5628cc28fe8c72e6fbb3ac05ed0ac55f504d2c)
+++ abi/include/_bits/decls.h	(revision 9fc58e32af670fae014980651086ee4bccd4836b)
@@ -61,4 +61,14 @@
 #endif  /* __cplusplus */
 
+/*
+ * __CONSTEXPR for headers shared by C and C++. Headers that are purely for
+ * C++ sources should use constexpr directly.
+ */
+#ifdef __cplusplus
+#define __CONSTEXPR constexpr
+#else
+#define __CONSTEXPR
+#endif
+
 #endif