THRIFT-4759:add thrift export to fix windows shared lib building (#1714)

* add THRIFT_EXPORT to fix windows shared lib building
* use shared libraries on vs2017 appveyor build, leave msvc2017 static link
diff --git a/appveyor.yml b/appveyor.yml
index 39aba4f..74c92f1 100755
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -39,7 +39,7 @@
    - PROFILE: MSVC2017
      PLATFORM: x64
      CONFIGURATION: Release
-     BUILD_SHARED_LIBS: OFF
+     BUILD_SHARED_LIBS: ON
      BOOST_VERSION: 1.67.0
      LIBEVENT_VERSION: 2.1.8
      PYTHON_VERSION: 3.6