Index: abi/include/_bits/decls.h
===================================================================
--- abi/include/_bits/decls.h	(revision dcef1127074a0bc848a38cc3831924924320c130)
+++ abi/include/_bits/decls.h	(revision 0b5aa1f073776eb55ebd8bb41de4d74d4ccb147c)
@@ -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