AppVeyor: Be explicit about build script to support msbuild 3.5

Error message was:
The project file "ALL_BUILD.vcproj" is in the ".vcproj" file format,
which MSBuild no longer supports. Please convert the project by
opening it in the Visual Studio IDE or running the conversion tool,
or use MSBuild 3.5 or earlier to build it.

appveyor.yml

@@ -186,9 +186,8 @@ before_build:
   - cd c:\projects\libexpat
   - cmake -G"%GENERATOR%" -DCMAKE_BUILD_TYPE=%CONFIGURATION% -DEXPAT_WARNINGS_AS_ERRORS=ON %CMAKE_ARGS% expat
 
-build:
-  parallel: true                  # enable MSBuild parallel builds
-  project: expat.sln              # path to Visual Studio solution or project
+build_script:
+  - msbuild "c:\projects\libexpat\expat.sln" /m /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
 
 test_script:
   - cp %CONFIGURATION%\\%EXPAT_DLL% tests\\%CONFIGURATION%\\