Problem: I/O thread poll poller build unsupported

Solution: use API_POLLER=poll instead of POLLER=poll

appveyor.yml

@@ -17,7 +17,7 @@ environment:
       ENABLE_CURVE: ON
     - platform: Win32
       configuration: Release
-      POLLER: poll
+      API_POLLER: poll
       WITH_LIBSODIUM: ON
       ENABLE_CURVE: ON
     - platform: Win32
@@ -93,7 +93,7 @@ before_build:
   - cmd: set LIBZMQ_BUILDDIR=C:\projects\build_libzmq
   - cmd: md "%LIBZMQ_BUILDDIR%"
   - cd "%LIBZMQ_BUILDDIR%"
-  - cmd: cmake -D CMAKE_INCLUDE_PATH="%SODIUM_INCLUDE_DIR%" -D CMAKE_LIBRARY_PATH="%SODIUM_LIBRARY_DIR%" -D WITH_LIBSODIUM="%WITH_LIBSODIUM%" -D ENABLE_DRAFTS="ON" -D ENABLE_CURVE="%ENABLE_CURVE%" -D POLLER="%POLLER%" -D CMAKE_C_FLAGS_RELEASE="/MT" -D CMAKE_C_FLAGS_DEBUG="/MTd" -D WITH_LIBSODIUM="%WITH_LIBSODIUM%" -D LIBZMQ_WERROR="ON" -G "%CMAKE_GENERATOR%" "%APPVEYOR_BUILD_FOLDER%"
+  - cmd: cmake -D CMAKE_INCLUDE_PATH="%SODIUM_INCLUDE_DIR%" -D CMAKE_LIBRARY_PATH="%SODIUM_LIBRARY_DIR%" -D WITH_LIBSODIUM="%WITH_LIBSODIUM%" -D ENABLE_DRAFTS="ON" -D ENABLE_CURVE="%ENABLE_CURVE%" -D API_POLLER="%API_POLLER%" -D CMAKE_C_FLAGS_RELEASE="/MT" -D CMAKE_C_FLAGS_DEBUG="/MTd" -D WITH_LIBSODIUM="%WITH_LIBSODIUM%" -D LIBZMQ_WERROR="ON" -G "%CMAKE_GENERATOR%" "%APPVEYOR_BUILD_FOLDER%"
 
 build:
   parallel: true