/Externals/liblzma/config.h #defines HAVE_PTHREAD_CONDATTR_SETCLOCK
which results in /Externals/liblzma/tuklib/mythread.h calling the
function pthread_condattr_setclock. This function doesn't exist in
macOS 10.12+, which are the only macOS versions Dolphin supports, but
inserting an include guard makes macOS use an alternate function.
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