-Werror,-Wconstexpr-not-const

...at least current trunk towards Clang 3.4 complains "'constexpr' non-static
member function will not be implicitly 'const' in C++1y; add 'const' to avoid a
change in behavior."

Change-Id: Id3d1af263040fdea2c2f5d0bc6a037d7a17184bd

boost/UnpackedTarball_boost.mk

@@ -43,6 +43,7 @@ boost_patches += boost.std.move.patch
 boost_patches += boost.wundef.patch
 boost_patches += boost.wunused.patch
 boost_patches += boost.wshadow.patch
+boost_patches += boost.wconstexpr-not-const.patch.0
 
 $(eval $(call gb_UnpackedTarball_UnpackedTarball,boost))
 

boost/boost.wconstexpr-not-const.patch.0

+--- boost/chrono/duration.hpp
++++ boost/chrono/duration.hpp
+@@ -663,36 +663,36 @@
+     template <class LhsDuration, class RhsDuration>
+     struct duration_eq
+     {
+-      BOOST_CONSTEXPR bool operator()(const LhsDuration& lhs, const RhsDuration& rhs)
++      BOOST_CONSTEXPR bool operator()(const LhsDuration& lhs, const RhsDuration& rhs) const
+         {
+             typedef typename common_type<LhsDuration, RhsDuration>::type CD;
+             return CD(lhs).count() == CD(rhs).count();
+         }
+     };
+ 
+     template <class LhsDuration>
+     struct duration_eq<LhsDuration, LhsDuration>
+     {
+-      BOOST_CONSTEXPR bool operator()(const LhsDuration& lhs, const LhsDuration& rhs)
++      BOOST_CONSTEXPR bool operator()(const LhsDuration& lhs, const LhsDuration& rhs) const
+         {
+             return lhs.count() == rhs.count();
+         }
+     };
+ 
+     template <class LhsDuration, class RhsDuration>
+     struct duration_lt
+     {
+-      BOOST_CONSTEXPR bool operator()(const LhsDuration& lhs, const RhsDuration& rhs)
++      BOOST_CONSTEXPR bool operator()(const LhsDuration& lhs, const RhsDuration& rhs) const
+         {
+             typedef typename common_type<LhsDuration, RhsDuration>::type CD;
+             return CD(lhs).count() < CD(rhs).count();
+         }
+     };
+ 
+     template <class LhsDuration>
+     struct duration_lt<LhsDuration, LhsDuration>
+     {
+-      BOOST_CONSTEXPR bool operator()(const LhsDuration& lhs, const LhsDuration& rhs)
++      BOOST_CONSTEXPR bool operator()(const LhsDuration& lhs, const LhsDuration& rhs) const
+         {
+             return lhs.count() < rhs.count();
+         }