${cryptopp_SOURCE_DIR} is defined as the place the source is downloaded to, so trying to make it download to ${cryptopp_SOURCE_DIR}/cryptopp isn’t the right thing to be trying to do. You can define the actual directory by specifying SOURCE_DIR as one of the FetchContent_Declare() arguments. Then you take on the full responsibility of where to put the source dir too (i.e. the full path), not just the name of the leaf directory.
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