checkout git submodules as PowerToys.sln prebuild action

deps/cxxopts.props

@@ -1,4 +1,5 @@
 <Project>
+	<Import Project="restore_git_submodules.props" Condition="'$(RestoreGitSubmodulesImported)' == ''" />
 	<ItemDefinitionGroup>
 		<ClCompile>
 			<AdditionalIncludeDirectories>$(MSBuildThisFileDirectory)cxxopts\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>

deps/restore_git_submodules.props

@@ -0,0 +1,12 @@
+<Project>
+  <PropertyGroup>
+    <RestoreGitSubmodulesImported>true</RestoreGitSubmodulesImported>
+  </PropertyGroup>
+  <Target Name="RestoreGitSubmodules" BeforeTargets="PrepareForBuild">
+      <Exec IgnoreExitCode="true"
+            EchoOff="true"
+            StandardOutputImportance="low"
+            StandardErrorImportance="low"
+            Command="git submodule update --init" />
+  </Target>	
+</Project>

deps/spdlog.props

@@ -1,4 +1,5 @@
 <Project>
+	<Import Project="restore_git_submodules.props" Condition="'$(RestoreGitSubmodulesImported)' == ''" />
 	<ItemDefinitionGroup>
 		<ClCompile>
 			<AdditionalIncludeDirectories>$(MSBuildThisFileDirectory)spdlog\include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>