Fix linker errors about __imp___CrtDbgReportW

We were linking against the release CRT but defining _DEBUG, which should only
be defined when using the debug CRT.
Make our Windows build settings actually take effect

We need to put them inside the target_defaults dictionary. Now that these
settings are turned on, we need to ignore some warnings that Chromium ignores.
Set up linker defaults for Windows

These match Chromium's defaults.
Merge pull request #18 from brightray/better-windows-defaults

Improve Windows default build settings