Add any preprocessor definitions passed using msbuild enviornment to

list of PreprocessorDefinitions

This is needed for passing preprocessors to msbuild for node build https://github.com/Microsoft/ChakraCore/commit/62fe85a08bb12244626a51456222113544c706d7

Build/Common.Build.props

@@ -9,6 +9,9 @@
 
       <PreprocessorDefinitions Condition="'$(RuntimeLib)'=='static_library'">%(PreprocessorDefinitions);USE_STATIC_RUNTIMELIB</PreprocessorDefinitions>
 
+      <!-- Add any preprocessor definitions passed using msbuild environment -->
+      <PreprocessorDefinitions Condition="'$(AdditionalPreprocessorDefinitions)'!=''">%(PreprocessorDefinitions);$(AdditionalPreprocessorDefinitions)</PreprocessorDefinitions>
+      
       <!-- /W4 -->
       <WarningLevel>Level4</WarningLevel>
       <!-- /WX -->