Enhance the Windows GN build

This removes most of the hard coded linker flags and puts them in the build config properly.

It now uses the Windows 8 SDK so that the base builds again. This is hardcoded, but GYP is hardcoded in the same way.

I renamed libxml2 to libxml and set the output name instead (this saves some typing, the output_name feature didn't exist when I wrote this).

Added a fastbuild flag with (nearly) the same meaning as GYP.

I parameterized a lot of the symbol settings, optimizations, and such in configs.

Fixed a bug where static libraries were not inherited if a target was marked external. We should only follow the external flag if we're using an external generator (this fixes the standalone build).

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/25772004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227298 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227298