diff --git a/include/__config b/include/__config
index ce39243c..f803c9a8 100644
--- a/include/__config
+++ b/include/__config
@@ -368,7 +368,14 @@ namespace std {
 #endif
 
 #define _LIBCPP_HAS_NO_TEMPLATE_ALIASES
+
+// constexpr was added to GCC in 4.6.
+#if _GNUC_VER < 406
 #define _LIBCPP_HAS_NO_CONSTEXPR
+// Can only use constexpr in c++11 mode.
+#elif !defined(__GXX_EXPERIMENTAL_CXX0X__) && __cplusplus < 201103L
+#define _LIBCPP_HAS_NO_CONSTEXPR
+#endif
 
 #define _NOEXCEPT throw()
 #define _NOEXCEPT_(x)
diff --git a/include/bitset b/include/bitset
index 4cc7dbda..8c278cc7 100644
--- a/include/bitset
+++ b/include/bitset
@@ -249,9 +249,9 @@ inline _LIBCPP_INLINE_VISIBILITY
 _LIBCPP_CONSTEXPR
 __bitset<_N_words, _Size>::__bitset(unsigned long long __v) _NOEXCEPT
 #ifndef _LIBCPP_HAS_NO_CONSTEXPR
-#if __SIZE_WIDTH__ == 64
+#if __SIZEOF_SIZE_T__ == 8
     : __first_{__v}
-#elif __SIZE_WIDTH__ == 32
+#elif __SIZEOF_SIZE_T__ == 4
     : __first_{__v, __v >> __bits_per_word}
 #else
 #error This constructor has not been ported to this platform