add appveyor config for VS2017

The 'Visual Studio 2017' image does not include VS2010, so we have to use
the 'Visual Studio 2015' image by default and only use the 2017 image for
msvc2017 compiler tests.

.appveyor.yml

@@ -20,6 +20,16 @@ environment:
       compiler: msvc2015
       backend: vs2015
 
+    - arch: x86
+      compiler: msvc2017
+      backend: ninja
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+
+    - arch: x86
+      compiler: msvc2017
+      backend: vs2015
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+
     - arch: x64
       compiler: msvc2015
       backend: ninja
@@ -28,6 +38,16 @@ environment:
       compiler: msvc2015
       backend: vs2015
 
+    - arch: x64
+      compiler: msvc2017
+      backend: ninja
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+
+    - arch: x64
+      compiler: msvc2017
+      backend: vs2015
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+
 platform:
   - x64
 
@@ -43,6 +63,7 @@ install:
   - cmd: echo Using Python at %MESON_PYTHON_PATH%
   - cmd: if %compiler%==msvc2010 ( call "C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\vcvarsall.bat" %arch% )
   - cmd: if %compiler%==msvc2015 ( call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" %arch% )
+  - cmd: if %compiler%==msvc2017 ( call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\Tools\VsDevCmd.bat" -arch=%arch% )
 
 build_script:
   - cmd: echo No build step.