support/dependencies: requires cmake >= 3.18

gerbera requires cmake 3.18 since version 1.10.0 and
https://github.com/gerbera/gerbera/commit/ac05976dda335ce1c28a5cc78e4e009a145e3559



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>