Fixed a compiler warning with MinGW toolchain

With -Wall set, MinGW warns about unused functions, and the
restart_process function was unused if the game was compiled using
something other than Visual Studio. The function is no longer compiled
in with non-Microsoft toolchains.

src/wesnoth.cpp

@@ -926,7 +926,7 @@ static void wesnoth_terminate_handler(int) {
 }
 #endif
 
-#ifdef _WIN32
+#if defined(_OPENMP) && _MSC_VER >= 1600
 static void restart_process(const std::vector<std::string>& commandline)
 {
 	wchar_t process_path[MAX_PATH];