msbuild: refactor to reduce rebuild events

* moves dolphin-specific settings out of Base.props
* creates exports.props for externals, allowing to easily import
  individual Externals
* corrects some cruft that accumulated and probably contributed
  to msbuild overbuilding

Externals/enet/enet.vcxproj

@@ -15,6 +15,11 @@
     <Import Project="$(VSPropsDir)ClDisableAllWarnings.props" />
   </ImportGroup>
   <PropertyGroup Label="UserMacros" />
+  <ItemDefinitionGroup>
+    <ClCompile>
+      <AdditionalIncludeDirectories>include;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
+    </ClCompile>
+  </ItemDefinitionGroup>
   <ItemGroup>
     <ClInclude Include="include\enet\callbacks.h" />
     <ClInclude Include="include\enet\enet.h" />