roblem: Visual Studio Toolset was incorrectly set in VCXPROJ files

Solution: Corrected Toolset setting where needed and inprove compilation speed
by adding defintion of WIN32_LEAN_AND_MEAN prior to any Windows specific
include files, which skips non-essential definitions during compilation.

src/precompiled.hpp

@@ -36,6 +36,7 @@
 #include "platform.hpp"
 
 #if defined ZMQ_HAVE_WINDOWS
+#define WIN32_LEAN_AND_MEAN		// speeds up compilation by removing rarely used windows definitions from headers
 #include "windows.hpp"
 #else
 #include <unistd.h>