ninja/win: Set /O2 compiler flag by default if no 'Optimization' specified

Follows behavior when using MSVS generators. If target ends up with no
'Optimization' option specified, MSVS generator will not add <Optimization>
element in the <ClCompile> parent. Visual Studio will still set /O2 compile
flag though.

Tested only in VS2012 Express.

BUG=https://code.google.com/p/gyp/issues/detail?id=361
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/23253003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1697 78cadc50-ecff-11dd-a971-7dbc132099af
2 files changed