Cleanup foo.h headers and __config to work in C

llvm-svn: 252274
diff --git a/libcxx/include/setjmp.h b/libcxx/include/setjmp.h
index ec4a5f6..464b4a5 100644
--- a/libcxx/include/setjmp.h
+++ b/libcxx/include/setjmp.h
@@ -34,8 +34,12 @@
 
 #include_next <setjmp.h>
 
+#ifdef __cplusplus
+
 #ifndef setjmp
 #define setjmp(env) setjmp(env)
 #endif
 
+#endif // __cplusplus
+
 #endif  // _LIBCPP_SETJMP_H