Fix a macro regression when using MSVC (#106)

Author: ivanka2012

CMakeLists.txt

@@ -42,11 +42,11 @@ target_include_directories(
           ${PROJECT_SOURCE_DIR}/trantor/net/inner
           $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/third_party/wepoll>)
 
-if(WIN32)
+if(MINGW)
   target_compile_definitions(
     ${PROJECT_NAME}
     PUBLIC -D_WIN32_WINNT=0x0601)
-endif(WIN32)
+endif(MINGW)
 
 set(TRANTOR_SOURCES
     trantor/utils/AsyncFileLogger.cc