Ignore MSVC linker warnings about empty object files

A number of object files in wxCore can end up being empty due to the
corresponding wxUSE_XXX options being set to 0. This is normal and
getting tons of warnings from MSVC during the build about it wasn't
helpful.

It turns out that these warnings can be suppressed by using the
undocumented, but described at

http://www.geoffchappell.com/studies/msvc/link/link/options/ignore.htm

"/ignore" option, so do this to finally get a clean build.

build/msw/wx_core.vcxproj

@@ -160,6 +160,7 @@
     </ResourceCompile>
     <Lib>
       <SuppressStartupBanner>true</SuppressStartupBanner>
+      <AdditionalOptions>/ignore:4221 %(AdditionalOptions)</AdditionalOptions>
     </Lib>
     <Bscmake>
       <OutputFile>$(OutDir)wx_$(wxCompilerPrefix)_$(ProjectName).bsc</OutputFile>
@@ -196,6 +197,7 @@
     <Lib>
       <OutputFile>$(OutDir)$(TargetName)$(TargetExt)</OutputFile>
       <SuppressStartupBanner>true</SuppressStartupBanner>
+      <AdditionalOptions>/ignore:4221 %(AdditionalOptions)</AdditionalOptions>
     </Lib>
     <Bscmake>
       <OutputFile>$(OutDir)wx_$(wxCompilerPrefix)_$(ProjectName).bsc</OutputFile>
@@ -234,6 +236,7 @@
     <Lib>
       <SuppressStartupBanner>true</SuppressStartupBanner>
       <TargetMachine>MachineX86</TargetMachine>
+      <AdditionalOptions>/ignore:4221 %(AdditionalOptions)</AdditionalOptions>
     </Lib>
     <Bscmake>
       <OutputFile>$(OutDir)wx_$(wxCompilerPrefix)_$(ProjectName).bsc</OutputFile>
@@ -271,6 +274,7 @@
     <Lib>
       <OutputFile>$(OutDir)$(TargetName)$(TargetExt)</OutputFile>
       <SuppressStartupBanner>true</SuppressStartupBanner>
+      <AdditionalOptions>/ignore:4221 %(AdditionalOptions)</AdditionalOptions>
     </Lib>
     <Bscmake>
       <OutputFile>$(OutDir)wx_$(wxCompilerPrefix)_$(ProjectName).bsc</OutputFile>
@@ -1502,4 +1506,4 @@
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">
   </ImportGroup>
-</Project>
+</Project>