meson: add configurable executable name
This change makes the name of the executable (usually calls) configurable and allows to drop a patch in Debian packaging (https://salsa.debian.org/DebianOnMobile-team/gnome-calls/-/blob/debian/master/debian/patches/0001-rename-binary.patch) renaming the executable to gnome-calls (calls was deemed a too generic name).
Edited by Evangelos Ribeiro Tzaras