Set -pch correctly from targets when not using PCH (#1093)

Author: davidmatson

nuget/Microsoft.Windows.CppWinRT.targets

@@ -804,7 +804,8 @@ $(XamlMetaDataProviderPch)
             Text="Please retarget to 10.0.17709.0 or later, or rename your PCH to 'pch.h'."/>
         <PropertyGroup Condition="('$(CppWinRTOverrideSDKReferences)' == 'true') or ('$(TargetPlatformVersion)' &gt; '10.0.17708.0')">
             <CppWinRTUsePrefixes Condition="'$(CppWinRTUsePrefixes)' == ''">true</CppWinRTUsePrefixes>
-            <CppWinRTPrecompiledHeader Condition="'$(CppWinRTPrecompiledHeader)' == ''">$(_PCH)</CppWinRTPrecompiledHeader>
+            <CppWinRTPrecompiledHeader Condition="'$(CppWinRTPrecompiledHeader)' == '' and '%(CLCompile.PrecompiledHeader)' != 'NotUsing'">$(_PCH)</CppWinRTPrecompiledHeader>
+            <CppWinRTPrecompiledHeader Condition="'$(CppWinRTPrecompiledHeader)' == '' and '%(CLCompile.PrecompiledHeader)' == 'NotUsing'">.</CppWinRTPrecompiledHeader>
         </PropertyGroup>
         <PropertyGroup>
             <CppWinRTCommandUsePrefixes Condition="'$(CppWinRTUsePrefixes)' == 'true'">-prefix</CppWinRTCommandUsePrefixes>