Check for libc++ before using it.

Fixes #731.
Changed HEAD of third_party/argparse (Create gitignore file).
Merge pull request #736 from dzhioev/master

Changed HEAD of third_party/argparse (Create gitignore file).
Try to cleanup files only if they stil exist

Fixes #738
Better handling of unknown extra conf files

Previously, we'd implicitly turn off future notices about unknown extra conf
files if we already raised one exception about it. This breaks when the user
ends up not receiving the "unknown extra conf, load?" message.

Now we only turn off the notice as a result of the user saying "don't load this"
so that if the first request fails to reach them, they'll get a second (and
third etc) request about it.

Fixes #615
Merge pull request #1 from Valloric/master

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