[Pull request] Use comptopt only when available

[Nick Felt]

Could I petition for this pull request to still be merged?

Original thread was here if this post doesn't thread properly:
https://lists.zx2c4.com/pipermail/password-store/2017-April/002869.html

I ran into the compopt errors when I upgraded pass recently, and they were
annoying to track down.  This tiny two-line change would fix that issue for
new pass users and avoid them having to deal with the same problem.  While
I recognize that using homebrew bash is a workaround, it's such a small
change to make this work out of the box with macOS system bash that it
seems worth doing.
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