This is an initial commit of constexpr support as proposed by Richard Smith.  This by no means completes constexpr support.  Indeed, it hardly scratches the surface.  All it does is lay the foundation in <__config> and changes those few places in the library that are already using that foundation.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@153856 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/algorithm b/include/algorithm
index f9c6843..68e989a 100644
--- a/include/algorithm
+++ b/include/algorithm
@@ -2712,8 +2712,8 @@
 
     result_type operator()();
 
-    static constexpr result_type min() {return _Min;}
-    static constexpr result_type max() {return _Max;}
+    static _LIBCPP_CONSTEXPR result_type min() {return _Min;}
+    static _LIBCPP_CONSTEXPR result_type max() {return _Max;}
 
     friend __rs_default __rs_get();
 };