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When exporting from a project (with install(EXPORT ...)), the `<PROJECT>Targets.cmake` file contains logic for computing the `_IMPORT_PREFIX` from its own location. This `_IMPORT_PREFIX` is then used in the `<PROJECT>Targets-<config>.cmake` file to generate the `IMPORTED_LOCATION_<CONFIG>`. The generation unconditionally appends a "/" to `_IMPORT_PREFIX` before appending the rest of the path. If `_IMPORT_PREFIX` is "/", then the `IMPORTED_LOCATION_<CONFIG>` properties all start with exactly two leading slashes ("//"). Exactly two leading slashes is a special case in POSIX file paths, such that its interpretation is left up to the implementation. This means that changing the path prefix from "/" to "//" should not be allowed. Since references to `_IMPORT_PREFIX` are always followed by a "/", simply check the value to replace "/" with "".
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