This does not convert the build side, or remove any of the checking it
does. We still need that for other callers of custom target. What we'll
do for those is add an internal interface that defaults things, then
we'll be able to have those callers do their own validation, and the
CustomTarget validation machinary can be removed.
Fixes#9096
Currently, EnvironmentVariablesObject is a strange
holder-that's-not-a-holder. This has implicaitons for things that expect
to get an EnvironmentVariables object, as we can't automatically
unholder it, and instead have to to manually do so. Now we can
automatically unholder it, which makes everything much nicer.
This is useful both from the perspective of optional functionality that
requires a module, and also as I continue to progress with Meson++,
which will probably not implement all of the modules that Meson itself
does.
This makes use of the new convertor and validator arguments, so that we
can check that the languages passed are in fact vaild, and then convert
the native boolean into a MachineChoice internally.
Dylan Baker
Tristan Partin
Laurin-Luis Lehning
Daniel Mensinger