Why does ClamAV / freshclam complain about missing digital signatures?

In order for ClamAV's freshclam to verify virus database updates' digital signatures, it requires the GMP Library (GNU Multiple Precision Arithmetic Library). Unfortunately, GMP does not come with OS X 10.3.x/10.4.x/10.5.x server and is not trivial to install. This is however not to be considered a major issue and you don't need to worry about it.