bpo-46778: Enable multiprocess compilation for source files when building on Windows (GH-31390)

Author: jkloth

PCbuild/pyproject.props

@@ -45,6 +45,7 @@
       <WholeProgramOptimization>true</WholeProgramOptimization>
       <ControlFlowGuard Condition="$(EnableControlFlowGuard) != ''">$(EnableControlFlowGuard)</ControlFlowGuard>
       <AdditionalOptions>/utf-8 %(AdditionalOptions)</AdditionalOptions>
+      <MultiProcessorCompilation>true</MultiProcessorCompilation>
     </ClCompile>
     <ClCompile Condition="$(Configuration) == 'Debug'">
       <InlineFunctionExpansion>OnlyExplicitInline</InlineFunctionExpansion>