VisualC: turn on multi-processor compilation.

This makes SDL2 build in 12 seconds, instead of 50, on my Windows
workstation, with Visual Studio.

VisualC/SDL/SDL.vcxproj

@@ -137,6 +137,7 @@
       <DebugInformationFormat>OldStyle</DebugInformationFormat>
       <OmitDefaultLibName>true</OmitDefaultLibName>
       <InlineFunctionExpansion>OnlyExplicitInline</InlineFunctionExpansion>
+      <MultiProcessorCompilation>true</MultiProcessorCompilation>
     </ClCompile>
     <ResourceCompile>
       <PreprocessorDefinitions>_DEBUG;%(PreprocessorDefinitions)</PreprocessorDefinitions>