std::forward not available when compiling with clang 3.0 c++0x

[viboes]

Recent modification on Boost.Test make it unusable with clang 3.0 c++0x due to std::forward missing or not found.

clang-darwin.compile.c++ ../../../bin.v2/libs/thread/test/test_thread.test/clang-darwin-3.0x/debug/threading-multi/test_thread.o
In file included from test_thread.cpp:16:
In file included from ../../../boost/test/unit_test.hpp:19:
In file included from ../../../boost/test/test_tools.hpp:328:
In file included from ../../../boost/test/tools/impl.hpp:25:
../../../boost/test/tools/assertion.hpp:278:21: error: no member named 'forward' in namespace 'std'
    : m_value( std::forward<T>(val) )
               ~~~~~^
../../../boost/test/tools/assertion.hpp:278:29: error: 'T' does not refer to a value
    : m_value( std::forward<T>(val) )
                            ^

This has as consequence that the authors developing mainly with this compiler and using Boost.Test to be unable to deliver new modifications.

[timblechmann]

proposed patch hopefully fixes the use of boost.test with c++11 builds

[Gennadiy Rozental]

(In [81178]) added header for std::forward; Fixes #6345