link warnings on MacOs
found during #1565 (closed)
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libkea-dhcp++.a(libkea_dhcp___la-iface_mgr_linux.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libkea-dhcp++.a(libkea_dhcp___la-iface_mgr_sun.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libkea-dhcp++.a(libkea_dhcp___la-iface_mgr_linux.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libkea-dhcp++.a(libkea_dhcp___la-iface_mgr_sun.o) has no symbols