VS2015: Ignore LNK4221 and properly force SSE off for Win32

This warning complains of files that don't define any new symbols
not being included in linking.
GCC doesn't seem to complain about those, so neither should VS.

Signed-off-by: Mark Harris <mark.hsj@gmail.com>
Closes https://github.com/xiph/opus/pull/34

win32/VS2015/opus.vcxproj

@@ -161,7 +161,11 @@
       <AdditionalIncludeDirectories>..\..\silk\fixed;..\..\silk\float;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
       <PreprocessorDefinitions Condition="'$(ConfigurationType)'=='DynamicLibrary'">DLL_EXPORT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <PreprocessorDefinitions Condition="'$(Configuration)'=='DebugDLL_fixed' or '$(Configuration)'=='ReleaseDLL_fixed'">FIXED_POINT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <AdditionalOptions Condition="'$(Platform)'=='Win32'">/arch:IA32 %(AdditionalOptions)</AdditionalOptions>
     </ClCompile>
+    <Lib>
+      <AdditionalOptions>/ignore:4221 %(AdditionalOptions)</AdditionalOptions>
+    </Lib>
     <PreBuildEvent>
       <Command>"$(ProjectDir)..\..\win32\genversion.bat" "$(ProjectDir)..\..\win32\version.h" PACKAGE_VERSION</Command>
       <Message>Generating version.h</Message>