The compiler may be installed as e.g. valac-0.48 to allow multiple versions to be installed.
I am unable to test this with the case when only
valac is present but I believe it will work.
meson: Support versioned valadoc and vapigen commands
meson: Make Vala optional
Default to Vala being enabled, with new 'vapi' option.
Fixes configuration failure if Vala is not installed: meson.build:1:0: ERROR: Could not execute Vala compiler "valac"