Problem: Appveyor build is slow. (zeromq#212)

* Problem: Appveyor Windows build is slow

Solution: use Appveyor caching for libzmq dependency

appveyor.yml

@@ -16,20 +16,24 @@ configuration:
     - Debug
 
 environment:
-    ZMQ_VERSION: 4.2.5
+    ZMQ_VER: 4.2.5
+
+cache:
+    - libzmq-%ZMQ_VER% -> appveyor.yml
 
 before_build:
-    - appveyor DownloadFile https://github.com/zeromq/libzmq/archive/v%ZMQ_VERSION%.zip
-    - 7z x v%ZMQ_VERSION%.zip >NUL
-    - cmake -H./libzmq-%ZMQ_VERSION% -BBuild-libzmq -DENABLE_DRAFTS=ON -DWITH_PERF_TOOL=OFF -DZMQ_BUILD_TESTS=OFF -DENABLE_CPACK=OFF -A%PLATFORM%
-    - cmake --build Build-libzmq
-    - cmake -H. -BBuild -DCMAKE_PREFIX_PATH=./Build-libzmq -A%PLATFORM%
+    - if not exist libzmq-%ZMQ_VER% (
+        appveyor DownloadFile https://github.com/zeromq/libzmq/archive/v%ZMQ_VER%.zip &&
+        7z x v%ZMQ_VER%.zip >NUL &&
+        cmake -H./libzmq-%ZMQ_VER% -Blibzmq-%ZMQ_VER%/build -DENABLE_DRAFTS=ON -DWITH_PERF_TOOL=OFF -DZMQ_BUILD_TESTS=OFF -DENABLE_CPACK=OFF -A%PLATFORM% &&
+        cmake --build libzmq-%ZMQ_VER%/build)
+    - cmake -H. -BBuild -DCMAKE_PREFIX_PATH=./libzmq-%ZMQ_VER%/build -A%PLATFORM%
 
 build:
     project: Build/cppzmq.sln
     verbosity: normal
 
 test_script:
-    - cp Build-libzmq/bin/%configuration%/libzmq*.dll Build/bin/%configuration%/
+    - cp libzmq-%ZMQ_VER%/build/bin/%configuration%/libzmq*.dll Build/bin/%configuration%/
     - cd Build
     - ctest -V -C %configuration%