`declare` allows initializer, but ignores it

[cspiel]

OMake allows a declare instruction to take an
initializer, but silently ignores it. The
documentation (see Section 5.7) does not
mention this "extension".

declare public.X = init

#  Uncomment the next line to force "unbound variable: ..." error.
#println($"+ X = <$(public.X)>")

public.X = assign
println($"+ X = <$(public.X)>")

IMHO it would be preferable to have a warning in
such cases, like, e.g. "initialization ignored".