tentative fix of appveyor script

.appveyor.yml

@@ -12,6 +12,17 @@ image:
   - Visual Studio 2015
   - Visual Studio 2017
 
+cache:
+  - C:\cmake-3.8.0-win32-x86
+
+install:
+  - ps: |
+      if (![IO.File]::Exists("C:\cmake-3.8.0-win32-x86\bin\cmake.exe")) {
+        Start-FileDownload 'https://cmake.org/files/v3.8/cmake-3.8.0-win32-x86.zip'
+        7z x -y cmake-3.8.0-win32-x86.zip -oC:\
+      }
+      $env:PATH="C:\cmake-3.8.0-win32-x86\bin;$env:PATH"
+
 before_build:
   - ps: |
       mkdir build