I've got nothing then. You've obviously got a debug build of libhunpell in the build/lib directory, but install wants to copy a release version of libhunspell that you
don't have. Clearly you have a debug build of Sigil built/installed if manually copying libhunspelld.so to the install directory allows Sigil to function.
I'm assuming, of course, that you've been able to successfully install your modified debug version of Sigil at some point in the past, so please stop me if I'm wrong.
If that's the case and you've wiped the cmake build directory and started from scratch and it
still doesn't work, then I'm forced to assume that you've indeed somehow changed the install portions of the main cmake script. However, since your git repo is hosed, you'll need to manually diff the main cmakelists.txt file against a known good copy somehow to find out.
How were you planning to submit a merge request to the main Sigil repo with your proposed modifications if your forked repo is under the impression that ALL the source files have been altered?