INSTALL(FILES ${man1} DESTINATION "man/man1") - installs uncompressed manpages
Found the problem on this project https://github.com/drawpile/Drawpile On FreeBSD, I first patched share/man/man1 -> man/man1 (because this is the standard location). The statement from the subject installs the uncompressed manpage.
Changing it to INSTALL(FILES {man1} DESTINATION "
{CMAKE_INSTALL_PREFIX}/man/man1") installs the compressed manpage.
The correct behavior: both should install the compressed manpage because cmake assumes ${CMAKE_INSTALL_PREFIX} when not set.