gitian: Add -O2 for win32 build

In the win32 gitian descriptor the C*FLAGS are being overridden on `configure`, which causes the built-in optimization flags to be removed. Add `-O2` manually (but not `-g` as we're doing nothing with the debug information).

Improves performance of win32 gitian builds.

As a bonus, fixes issue #3494 (see this comment: https://github.com/bitcoin/bitcoin/pull/3497#issuecomment-31981316 ).