ARROW-4290: [C++/Gandiva] Support detecting correct LLVM version in Homebrew
ARROW-4290: [C++/Gandiva] Support detecting correct LLVM version in Homebrew

Author: Korn, Uwe <Uwe.Korn@blue-yonder.com>
Author: Uwe L. Korn <uwelk@xhochy.com>

Closes #3429 from xhochy/ARROW-4290 and squashes the following commits:

4617b2f8 <Uwe L. Korn> Simplify check for brew
1746e928 <Korn, Uwe> ARROW-4290:  Support detecting correct LLVM version in Homebrew

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