There is no specification or anything saying that the Libs var in .pc files only has to carry one single Library flag. Hence the generated efreet_mime.pc file is completly valid, maybe there is an issue in the cmake config files - i don't know. However, this is not related to the generated .pc file, Ecore avahi, edje, [...] all of them have -lm in the Libs: *list* of flags.
That's what I said, probably a cmake issue. ;)
Mainly because other efl based applications I've compiled have found this lib just fine.