enable MSVC Standards Conformance (/permissive-)

...for all projects except wxWidgets. it includes WinSDK files that do not
support /permissive- yet; and the only way around this is to disable the
flag on project level.

Externals/wxWidgets3/build/msw/wx_base.vcxproj

@@ -1365,7 +1365,13 @@
     <Import Project="..\..\..\..\Source\VSProps\WXWOverrides.props" />
     <Import Project="..\..\..\..\Source\VSProps\ClDisableAllWarnings.props" />
   </ImportGroup>
-  <PropertyGroup Label="UserMacros" />
+  <PropertyGroup Label="UserMacros">
+    <!--
+    wxWidgets includes Windows SDK header files that do not support /permissive- yet, which would fail the build.
+    Disable the flag until this is sorted out (it probably doesn't matter too much for Externals anyways.)
+    -->
+    <SupportsStandardConformance>false</SupportsStandardConformance>
+  </PropertyGroup>
   <ItemDefinitionGroup Condition="'$(Configuration)'=='Debug'">
     <ClCompile>
       <PreprocessorDefinitions>__WXDEBUG__;%(PreprocessorDefinitions)</PreprocessorDefinitions>

Source/VSProps/Base.props

@@ -80,6 +80,8 @@
       <RuntimeTypeInfo>false</RuntimeTypeInfo>
       <MinimalRebuild>false</MinimalRebuild>
       <MultiProcessorCompilation>true</MultiProcessorCompilation>
+      <!--Enable Standard Conformance (unless the project does not support it)-->
+      <AdditionalOptions Condition="'$(SupportsStandardConformance)' != 'false'">/permissive- %(AdditionalOptions)</AdditionalOptions>
       <!--Enforce some behaviors as standards-conformant when they don't default as such-->
       <AdditionalOptions>/Zc:inline /Zc:rvalueCast /volatile:iso %(AdditionalOptions)</AdditionalOptions>
       <!--Enable detailed debug info-->