kvmtool: Makefile: improve warning on missing optional packages

Currently there is a warning when the (highly optional) GTK3 support
cannot be compiled in. Actually many may consider GTK3 the least
interesting of the optional packages, so replace this rather verbose
warning with a brief list of not found optional packages: This will
give a much nicer output like this:

Makefile:291: Skipping optional libraries: bfd GTK3 vncserver aio

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>