rfe: please add @PLAINNAME0@

[stsp]

I have a custom_target that depends on a few other custom_targets. When I use @PAINNAME@, I get this:

src/meson.build:88:7: ERROR: custom_target: output cannot contain "@PLAINNAME@"
or "@BASENAME@" when there is more than one input (we can't know which to use)

Ok but @PLAINNAME0@ is unsupported, and custom_tgt seems to have no name() member for a manual extraction.

Please add @PLAINNAME0@ or name() member for custom_tgt, or, preferably, both of them. :)

[bruchar1]

Relates to ##10498

[stsp]

Good point. From these patches it can be seen how @PLAINNAME0@ support can be trivially added.