CMake needs help finding gettext libintl libraries in environments with non-standard GCC installs.
Usually system introspection happens in the top-level CMakeLists.txt, but test-suite allows to selectively include subdirs, so it is a good location to do here.
If ClamAV is using gettext only, maybe use FindGettext instead?
Please use find_package (also see https://cmake.org/cmake/help/latest/manual/cmake-modules.7.html#find-modules)
The CMake documentation says to use FindIntl for the gettext libintl libraries
but use FindGettext for tools (msmerge, msgfmt, etc.).
ClamAV is only using the former.
I would have done an unequal: if (NOT TARGET_OS STREQUAL "Darwin"). libIntl is part of the GNU user space environment, not the kernel, e.g. also apply to GNU/kFreeBSD.
More generally, IMHO cmake should have abstracted over this, for platforms that don't need libIntl.