THRIFT-4081: appveyor retooling - added mingw64 build as a second job to the CI build process

This closes #1205
diff --git a/lib/cpp/CMakeLists.txt b/lib/cpp/CMakeLists.txt
index d07b400..9a1ef10 100755
--- a/lib/cpp/CMakeLists.txt
+++ b/lib/cpp/CMakeLists.txt
@@ -68,7 +68,7 @@
 endif()
 
 
-if (WIN32)
+if (WIN32 AND NOT MSYS)
     list(APPEND thriftcpp_SOURCES
         src/thrift/windows/TWinsockSingleton.cpp
         src/thrift/windows/SocketPair.cpp