Windows CI: use 32 and 64 bit for AppVeyor builds

Add 64 bit and always build with default calling conventions, to
avoid trying to build with stdcall on amd64.

appveyor.yml

@@ -2,6 +2,12 @@ version: '{build}'
 branches:
   only:
   - master
+environment:
+  matrix:
+  - GENERATOR: "Visual Studio 11"
+    ARCH: 32
+  - GENERATOR: "Visual Studio 11 Win64"
+    ARCH: 64
 build_script:
 - ps: >-
     choco install cmake
@@ -12,7 +18,7 @@ build_script:
 
     cd build
 
-    cmake -D ENABLE_TRACE=ON -D BUILD_CLAR=ON -D STDCALL=ON .. -G"Visual Studio 11"
+    cmake -D ENABLE_TRACE=ON -D BUILD_CLAR=ON .. -G"$env:GENERATOR"
 
     cmake --build . --config RelWithDebInfo
 test_script: