Clang update.sh: always clear CMake's cache, and only set MACOSX_DEPLOYMENT_TARGET when needed

Clang update.sh: always clear CMake's cache, and only set MACOSX_DEPLOYMENT_TARGET when needed

Previously it was possible to run into situations where CMake would fail,
write a partial CMakeCache.txt, and on the next run of update.sh, we would
think the build was already configured and skip the CMake step.

This patch removes that problem by always clearing the cache and forcing
a full re-run of CMake. This also has the benefit that if the path to the
compiler or a compiler flag changes, the build will get updated.

Note that doing a full re-run of CMake doesn't mean doing a full re-build.

Also, we should only be setting MACOSX_DEPLOYMENT_TARGET when doing bootstrap
builds. This patch fixes that too.

BUG=415670
Committed: https://crrev.com/835667440fd8753d9b0378470fa74f273b5b2cad
Cr-Commit-Position: refs/heads/master@{#296771}

[hans, 2014-09-25 18:29:20]

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

[hans, 2014-09-25 18:29:20]

Please take a look.

[Nico, 2014-09-25 18:47:12]

lgtm

[hans, 2014-09-25 18:48:55]

The CQ bit was checked by hans@chromium.org

[commit-bot: I haz the power, 2014-09-25 18:50:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/603943003/1

[commit-bot: I haz the power, 2014-09-25 19:51:53]

Committed patchset #1 (id:1) as dd808507ee580ddb7d5ba572cc8a3ee8c95e0b9e

[commit-bot: I haz the power, 2014-09-25 19:52:25]

Patchset 1 (id:??) landed as
https://crrev.com/835667440fd8753d9b0378470fa74f273b5b2cad
Cr-Commit-Position: refs/heads/master@{#296771}