Buold fix of devel/boost-libs

[Ryo ONODERA <ryo_on%yk.rim.or.jp@localhost>]

Hi,

/usr/include/g++/bits/c++config.h from GCC 4.8.3 in NetBSD/amd64 current base
 does not define _GLIBCXX_HAVE_GTHR_DEFAULT.
And devel/boost-libs' thread support is disabled under NetBSD/amd64 current.
I want to commit the following patch to meta-pkgs/boost.
OK to commit? Binary under NetBSD/amd64 6.* is not changed.

Thank you.


$NetBSD$

* Fix build under NetBSD with GCC 4.8.3 in base
  GCC 4.8.3 does not define _GLIBCXX_HAVE_GTHR_DEFAULT
  in /usr/include/g++/bits/c++config.h.

--- boost/config/stdlib/libstdcpp3.hpp.orig     2014-03-08 14:33:38.000000000 
+0000
+++ boost/config/stdlib/libstdcpp3.hpp
@@ -36,7 +36,8 @@
         || defined(_GLIBCXX__PTHREADS) \
         || defined(_GLIBCXX_HAS_GTHREADS) \
         || defined(_WIN32) \
-        || defined(_AIX)
+        || defined(_AIX) \
+       || defined(__NetBSD__)
       //
       // If the std lib has thread support turned on, then turn it on in Boost
       // as well.  We do this because some gcc-3.4 std lib headers define 
_REENTANT