clang: Only explicitly call configure if it's never been called.
clang: Only explicitly call configure if it's never been called.

If it has been called, the clang build will re-run it if necessary. And running
configure when nothing has changed causes several things to be rebuilt.

On my local machine, this brings the time this script takes if nothing changed
from 2:50m to 12s.

Idea by evan.

BUG=72609
TEST=none

Review URL: http://codereview.chromium.org/6485011

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@74542 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser