fix incorrect libstdc++ from GCC >= 5.1 check

base/bit_cast.h

Index: base/bit_cast.h
diff --git a/base/bit_cast.h b/base/bit_cast.h
index ad38238beb5c08902691dc3a9c8a67784714595c..645bfef003a5fe8d8c869cafd45f3c4d9d64e68e 100644
--- a/base/bit_cast.h
+++ b/base/bit_cast.h
@@ -64,8 +64,7 @@ inline Dest bit_cast(const Source& source) {
   static_assert(sizeof(Dest) == sizeof(Source),
                 "bit_cast requires source and destination to be the same size");
 
-#if ((defined(__GLIBCXX__) && (__GLIBCXX__ >= 20150422)) || \
-     defined(_LIBCPP_VERSION))
+#if (__GNUC__ > 5 || (__GNUC__ == 5 && __GNUC_MINOR__ >= 1))

[Nico, 2016/04/06 00:39:40]

this drops the _libcpp_version check; that's probably not intentional?

also, this breaks clang with libstdc++ too, no?

[Mostyn Bramley-Moore, 2016/04/06 07:54:23]

Right- added this back.

   // GCC 5.1 contains the first libstdc++ with is_trivially_copyable.
   // Assume libc++ Just Works: is_trivially_copyable added on May 13th 2011.
   static_assert(std::is_trivially_copyable<Dest>::value,