Rename UNICODE to protobuf_UNICODE

appveyor.bat

@@ -10,7 +10,7 @@ goto :error
 echo Building C++
 mkdir build_msvc
 cd build_msvc
-cmake -G "%generator%" -Dprotobuf_BUILD_SHARED_LIBS=%BUILD_DLL% -DUNICODE=%UNICODE% ../cmake
+cmake -G "%generator%" -Dprotobuf_BUILD_SHARED_LIBS=%BUILD_DLL% -Dprotobuf_UNICODE=%UNICODE% ../cmake
 msbuild protobuf.sln /p:Platform=%vcplatform% /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll" || goto error
 cd %configuration%
 tests.exe || goto error

cmake/CMakeLists.txt

@@ -159,9 +159,9 @@ else (MSVC)
   set(LIB_PREFIX)
 endif (MSVC)
 
-if (UNICODE)
+if (protobuf_UNICODE)
   add_definitions(-DUNICODE -D_UNICODE)
-endif (UNICODE)
+endif (protobuf_UNICODE)
 
 include(libprotobuf-lite.cmake)
 include(libprotobuf.cmake)