[C++] Search for flatbuffers in <root>/lib64

[asfimport]

Newer versions of flatbuffers install into <root>/lib64. We also should look there for them.

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

PRs and other links:

• GitHub Pull Request #2237

_{Note: This issue was originally created as ARROW-2823. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
Issue resolved by pull request 2237
#2237