C++: Xcode 9 is not correctly detected

[asfimport]

See ray-project/ray#1000

Reporter: Uwe Korn / @xhochy
Assignee: Uwe Korn / @xhochy

PRs and other links:

• GitHub Pull Request #1121

_{Note: This issue was originally created as ARROW-1591. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
PR: #1121

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 1121
#1121