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 not doing anything with the
debug information).

Improves performance of win32 gitian builds.

As a bonus, fixes issue #3497.
Merge pull request #3503

0d512a9 gitian: Add -O2 for win32 build (Wladimir J. van der Laan)