[C++][GLib] Fail to build with Xcode 12.0.1

[asfimport]

Reporter: Terence Honles / @terencehonles
Assignee: Terence Honles / @terencehonles

PRs and other links:

• GitHub Pull Request #8584
• GitHub Pull Request #8587

_{Note: This issue was originally created as ARROW-10490. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 8587
#8587