Decrease Visual Studio warning level to W3

... to reduce the Appveyor build time.

Author: ldorau

tests/CMakeLists.txt

@@ -36,7 +36,7 @@ add_check_whitespace(tests-common ${CMAKE_CURRENT_SOURCE_DIR}/common/*.*pp)
 add_check_whitespace(tests-cmake ${CMAKE_CURRENT_SOURCE_DIR}/CMakeLists.txt)
 
 if(MSVC_VERSION)
-	add_flag(-W4)
+	add_flag(-W3)
 else()
 	add_flag(-Wall)
 endif()