Fix compilation under AppleClang

Summary:
Broken since we started to treat Clang and AppleClang differently (with
the switch to CMake 3.0)

FIXED-IN: v5.34.0
BUG: 377933

Reviewers: apol, rjvbb

Reviewed By: apol, rjvbb

Subscribers: Frameworks, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D5379