Generate an early-return guard in target Export files.
Previously it was necessary for writers of Config files which incude exported target files to write the guard themselves, but this was not immediately obvious or documented. Options for them would be to use a variable, or an INHERITED directory property in an effort to avoid accidental name clashes in all contexts in which find_package can be used. Getting this right requires boiler plate code, so generate a simpler check automatically instead.
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