Thread: provide physical concurrency conditional until it no regression is introduced.

[SVN r85926]

src/win32/thread.cpp

@@ -417,6 +417,7 @@ namespace boost
         return info.dwNumberOfProcessors;
     }
 
+#ifdef BOOST_THREAD_PROVIDES_PHYSICAL_CONCURRENCY
     unsigned thread::physical_concurrency() BOOST_NOEXCEPT
     {
         unsigned cores = 0;
@@ -438,6 +439,7 @@ namespace boost
         }
         return cores;
     }
+#endif
 
     thread::native_handle_type thread::native_handle()
     {