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When a user disables a bundled image-codec dependency (e.g. BUILD_ZLIB=OFF) to supply an external build but find_package() cannot locate it, OpenCVFindLibsGrfmt.cmake silently falls back to compiling the bundled 3rdparty version. As reported in the issue, this is misleading: the editable ZLIB_LIBRARY_DEBUG/RELEASE cache entries have no effect after the fallback and nothing points the user at the real knob (ZLIB_ROOT / ZLIB_DIR). Emit a message(WARNING) in each fallback branch, guarded by NOT BUILD_<lib>, so the built-in fallback is no longer silent when the user explicitly opted out of the bundled build. The warning names the correct search variables for that library and points to the CMake find_package / Find<lib> documentation. Applied consistently to the zlib, libjpeg, libtiff, libpng and libwebp fallback branches. The existing fallback behavior is intentionally left unchanged, so configurations that rely on the built-in fallback continue to build. Fixes #26746
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