Fix quoting of preprocessor-definition.

Summary:
If the LIBEXECDIR contains spaces (as it does specifically on the CI
systems) then this needs one extra layer of quoting to protect it from the
shell.

Test Plan: Build on the CI systems

Reviewers: dfaure, bcooksley

Reviewed By: bcooksley

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5713