fix compilation with cygwin

The variant of std::ifstream with a wide character file name is only
available when using Visual Studio but not when using Cygwin.

Author: Daniel Kroening

src/goto-cc/cl_message_handler.cpp

@@ -42,7 +42,7 @@ void cl_message_handlert::print(
 
   if(full_path.has_value() && !line.empty())
   {
-#ifdef _WIN32
+#ifdef _MSC_VER
     std::ifstream in(widen(full_path.value()));
 #else
     std::ifstream in(full_path.value());

src/goto-cc/gcc_message_handler.cpp

@@ -66,7 +66,7 @@ void gcc_message_handlert::print(
     const auto file_name = location.full_path();
     if(file_name.has_value() && !line.empty())
     {
-#ifdef _WIN32
+#ifdef _MSC_VER
       std::ifstream in(widen(file_name.value()));
 #else
       std::ifstream in(file_name.value());