Adding macro to automated nuget package for XP.

Author: stgates

Release/src/build/package_info.xml

@@ -26,6 +26,7 @@
     <ItemDefinitionGroup>
       <ClCompile>
         <AdditionalOptions Condition="'$(PlatformToolset)'=='v140'">/d2notypeopt %(AdditionalOptions)</AdditionalOptions>
+        <PreprocessorDefinitions Condition="'$(PlatformToolset)' == 'v120_xp'">CPPREST_TARGET_XP;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       </ClCompile>
     </ItemDefinitionGroup>
   </msbuild_hook>