Don't link against symbols that only exist on newer macOS versions

Summary:
Should fix the macOS build in T9574

See also https://github.com/Homebrew/homebrew-core/issues/2674 and
https://github.com/Homebrew/homebrew-core/issues/3727 and

Test Plan: Autoconf output for liblzma no longer indicates clock_gettime availability

Reviewers: vonreth, bcooksley, fazevedo, dschmidt

Reviewed By: vonreth

Differential Revision: https://phabricator.kde.org/D15374