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As was done in dc771113 for `support-files/CMakeLists.txt`
Do not rely on existence of `CMakeFiles/${target}.dir` directory
existence. It is not there for custom targets in Ninja build.

This regression was introduced in #1131 which likely copied the pattern
from e79e8406 before that regression was addressed in dc771113.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services.