clang packager/win: Check that GnuWin32 is in the path at the start.

clang packager/win: Check that GnuWin32 is in the path at the start.

It's needed to run tests, and I prefer getting an error message
immediately over getting one 17 minutes in, when the build is done
and tests start running.

BUG=82385
Committed: https://crrev.com/fc8254be282454c26a228545fca479e3b257daaa
Cr-Commit-Position: refs/heads/master@{#349932}

[Nico, 2015-09-19 04:54:05]

thakis@chromium.org changed reviewers:
+ hans@chromium.org

[hans, 2015-09-21 15:14:24]

lgtm

Huh, I was sure we had added this before.

[Nico, 2015-09-21 15:18:06]

The CQ bit was checked by thakis@chromium.org

[commit-bot: I haz the power, 2015-09-21 15:18:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1351143006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1351143006/1

[commit-bot: I haz the power, 2015-09-21 15:35:32]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-21 15:36:54]

Patchset 1 (id:??) landed as
https://crrev.com/fc8254be282454c26a228545fca479e3b257daaa
Cr-Commit-Position: refs/heads/master@{#349932}