msvc: Use single build target for dealing with generated files

Remove some duplication in the code for generating
qstrdefs.generated.h and py-version.h

windows/msvc/genhdr.targets

@@ -7,7 +7,6 @@
   </Target>
 
   <PropertyGroup>
-    <SrcDir>$(PyBaseDir)py\</SrcDir>
     <DestDir>$(PyBuildDir)genhdr\</DestDir>
   </PropertyGroup>
 
@@ -18,19 +17,18 @@
   <!--see py/py.mk under #qstr data-->
   <Target Name="MakeQstrData" DependsOnTargets="MakeDestDir">
     <PropertyGroup>
+      <PySrcDir>$(PyBaseDir)py\</PySrcDir>
       <PreProc>$(DestDir)qstrdefs.preprocessed.h</PreProc>
       <QstrDefs>$(PyBaseDir)unix\qstrdefsport.h</QstrDefs>
       <DestFile>$(DestDir)qstrdefs.generated.h</DestFile>
+      <TmpFile>$(DestFile).tmp</TmpFile>
     </PropertyGroup>
     <ItemGroup>
       <PyIncDirs Include="$(PyIncDirs)"/>
     </ItemGroup>
-    <Exec Command="cl /nologo /I@(PyIncDirs, ' /I') /Fi$(PreProc) /P $(SrcDir)qstrdefs.h"/>
-    <Exec Command="python $(SrcDir)makeqstrdata.py $(PreProc) $(QstrDefs) > $(DestFile).tmp"/>
-    <Exec Command="fc /B $(DestFile).tmp $(DestFile) > NUL 2>&amp;1" IgnoreExitCode="true">
-      <Output TaskParameter="ExitCode" PropertyName="FilesDiffer" />
-    </Exec>
-    <Copy SourceFiles="$(DestFile).tmp" DestinationFiles="$(DestFile)" Condition="'$(FilesDiffer)'!='0'"/>
+    <Exec Command="cl /nologo /I@(PyIncDirs, ' /I') /Fi$(PreProc) /P $(PySrcDir)qstrdefs.h"/>
+    <Exec Command="python $(PySrcDir)makeqstrdata.py $(PreProc) $(QstrDefs) > $(TmpFile)"/>
+    <MSBuild Projects="$(MSBuildThisFileFullPath)" Targets="CopyFileIfDifferent" Properties="SourceFile=$(TmpFile);DestFile=$(DestFile)"/>
   </Target>
 
   <!--see py/py-version.sh-->
@@ -55,6 +53,7 @@
     </PropertyGroup>
     <PropertyGroup>
       <DestFile>$(DestDir)py-version.h</DestFile>
+      <TmpFile>$(DestFile).tmp</TmpFile>
     </PropertyGroup>
     <ItemGroup>
       <Lines Include="// This file was generated by $([System.IO.Path]::GetFileName(`$(MsBuildThisFile)`))"/>
@@ -62,11 +61,19 @@
       <Lines Include="#define MICROPY_GIT_HASH &quot;$(GitHash)&quot;"/>
       <Lines Include="#define MICROPY_BUILD_DATE &quot;$([System.DateTime]::Now.ToString(`yyyy-MM-dd`))&quot;"/>
     </ItemGroup>
-    <WriteLinesToFile Lines="@(Lines)" File="$(DestFile).tmp" Overwrite="true"/>
-    <Exec Command="fc /B $(DestFile).tmp $(DestFile) > NUL 2>&amp;1" IgnoreExitCode="true">
+    <WriteLinesToFile Lines="@(Lines)" File="$(TmpFile)" Overwrite="true"/>
+    <MSBuild Projects="$(MSBuildThisFileFullPath)" Targets="CopyFileIfDifferent" Properties="SourceFile=$(TmpFile);DestFile=$(DestFile)"/>
+  </Target>
+
+  <!--Copies SourceFile to DestFile only if SourceFile's content differs from DestFile's.
+      We use this to 'touch' the generated files only when they are really newer
+      so a build is only triggered if the generated content actually changed,
+      and not just because the file date changed since the last build-->
+  <Target Name="CopyFileIfDifferent">
+    <Exec Command="fc /B $(SourceFile) $(DestFile) > NUL 2>&amp;1" IgnoreExitCode="true">
       <Output TaskParameter="ExitCode" PropertyName="FilesDiffer" />
     </Exec>
-    <Copy SourceFiles="$(DestFile).tmp" DestinationFiles="$(DestFile)" Condition="'$(FilesDiffer)'!='0'"/>
+    <Copy SourceFiles="$(SourceFile)" DestinationFiles="$(DestFile)" Condition="'$(FilesDiffer)'!='0'"/>
   </Target>
 
 </Project>