AppVeyor: use parallel builds

My group now has a subscription to AppVeyor pro, which also permits
running parallel builds on the open source projects.

.appveyor.yml

@@ -3,6 +3,8 @@ image:
 - Visual Studio 2017
 - Visual Studio 2015
 test: off
+build:
+  parallel: true
 platform:
 - x64
 - x86
@@ -35,6 +37,6 @@ install:
 build_script:
 - cmake -G "%CMAKE_GENERATOR%" -A "%CMAKE_ARCH%" -DPYBIND11_WERROR=ON
 - set MSBuildLogger="C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
-- cmake --build . --config Release --target pytest -- /v:m /logger:%MSBuildLogger%
-- cmake --build . --config Release --target test_cmake_build -- /v:m /logger:%MSBuildLogger%
+- cmake --build . --config Release --target pytest -- /v:m /m /logger:%MSBuildLogger%
+- cmake --build . --config Release --target test_cmake_build -- /v:m /m /logger:%MSBuildLogger%
 on_failure: if exist "tests\test_cmake_build" type tests\test_cmake_build\*.log