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..1e79db1457bb34b50fe7310b8425ee4ef28a152f 100644
--- a/base/bit_cast.h
+++ b/base/bit_cast.h
@@ -64,7 +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)) || \

[Mostyn Bramley-Moore, 2016/04/05 22:13:48]

This is the value from G++ 4.8, I believe.

+#if ((defined(__GLIBCXX__) && (__GLIBCXX__ >= 20151010ul)) || \

[Mostyn Bramley-Moore, 2016/04/05 22:13:48]

This is actually the value from G++ 5.2, just a placeholder until I can find the
value corresponding to 5.1.

[JF, 2016/04/05 22:21:29]

IIRC the docs are:

From: https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html

  Incremental bumping of a library pre-defined macro. For releases before 3.4.0,
the macro is __GLIBCPP__. For later releases, it's __GLIBCXX__. (The libstdc++
project generously changed from CPP to CXX throughout its source to allow the
"C" pre-processor the CPP macro namespace.) These macros are defined as the date
the library was released, in compressed ISO date format, as an unsigned long.

  This macro is defined in the file "c++config" in the
"libstdc++-v3/include/bits" directory. (Up to GCC 4.1.0, it was changed every
night by an automated script. Since GCC 4.1.0, it is the same value as
gcc/DATESTAMP.)


And from: https://gcc.gnu.org/releases.html

  GCC 5.1	April 22, 2015


It sounds like that doesn't work for you?

[Mostyn Bramley-Moore, 2016/04/05 22:26:05]

gcc/DATESTAMP in the GCC 4.8.5 tarball is 20150623, larger than the value from
the GCC 5.1 tarball.

      defined(_LIBCPP_VERSION))
   // GCC 5.1 contains the first libstdc++ with is_trivially_copyable.
   // Assume libc++ Just Works: is_trivially_copyable added on May 13th 2011.