Revise the test from commit 9433755e5d (FindBacktrace: Add imported library, 2024-04-06) to work when compiled as C90.
9433755e5d
This is to avoid (a future version of) Qt from having to wrap FindBacktrace like [1]. [1] https://code.qt.io/cgit/qt/qtbase.git/tree/cmake/FindWrapBacktrace.cmake