[builds/windows] Improve `DlgCopy` target.

Fixes #1113.

* builds/windows/vc2010/freetype.vcxproj: Run `DlgCopy` conditionally
and decouple it from `Build`.
diff --git a/builds/windows/vc2010/freetype.vcxproj b/builds/windows/vc2010/freetype.vcxproj
index aa8243f..dfe7735 100644
--- a/builds/windows/vc2010/freetype.vcxproj
+++ b/builds/windows/vc2010/freetype.vcxproj
@@ -11,7 +11,7 @@
   or with different appropriate switches. It also works with Visual Studio.

   Additional customization can be made in `freetype.user.props`.

 -->

-<Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

+<Project DefaultTargets="DlgCopy;Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

   <ItemGroup Label="ProjectConfigurations">

     <ProjectConfiguration Include="Debug|Win32">

       <Configuration>Debug</Configuration>

@@ -508,7 +508,7 @@
     <DlgSrc Include="..\..\..\subprojects\dlg\src\dlg\dlg.c" />

     <DlgDst Include="..\..\..\src\dlg\dlg.c" />

   </ItemGroup>

-  <Target Name="DlgCopy" Inputs="@(DlgSrc)" Outputs="@(DlgDst)" BeforeTargets="PrepareForBuild">

+  <Target Name="DlgCopy" Inputs="@(DlgSrc)" Outputs="@(DlgDst)" Condition="Exists('..\..\..\subprojects\dlg\.git')">

     <Copy SourceFiles="@(DlgSrc)" DestinationFiles="@(DlgDst)" SkipUnchangedFiles="true" />

   </Target>

   <Target Name="AfterBuild">