correct workaround for missing thread support

the Workaround from MeganZ (I suppose you refere to this one: https://github.com/meganz/mingw-std-threads) hast some flaws: <thread> is not available, <phread.h> is not available and EPROTO is already defined (but with 53 instead of 71)

CMakeLists.txt

@@ -109,6 +109,7 @@ if(WIN32)
 
     if(MINGW)
         if(NANA_CMAKE_ENABLE_MINGW_STD_THREADS_WITH_MEGANZ)
+            add_definitions(-DSTD_THREAD_NOT_SUPPORTED)
             add_definitions(-DNANA_ENABLE_MINGW_STD_THREADS_WITH_MEGANZ)
         endif(NANA_CMAKE_ENABLE_MINGW_STD_THREADS_WITH_MEGANZ)
     endif(MINGW)

include/nana/std_mutex.hpp

@@ -8,12 +8,12 @@
 #include <windows.h>
 #include <mutex>
 #include <condition_variable>
-#include <thread>
-#include <pthread.h>
+//#include <thread>
+//#include <pthread.h>
 #include <errno.h>
 #include <cstdio>
 // http://lxr.free-electrons.com/source/include/uapi/asm-generic/errno.h#L53
-#define EPROTO          71      /* Protocol error */
+//#define EPROTO          71      /* Protocol error */
 #include <mingw.thread.h>
 #include <mingw.mutex.h>
 #else