`add_halide_library` should complain if Halide_TARGET contains a comma

[alexreinking]

In CMake, Halide_TARGET expects a normal CMake list of targets, so that means using ; instead of , for multi-target specification. We should warn users if a , appears in that variable since it is never correct.

attn: @abadams

[steven-johnson]

I don't understand. What is 'a normal CMake list of targets'?

[alexreinking]

I mean a (normal CMake list) (of Halide target strings). CMake lists are semicolon-separated strings.

[steven-johnson]

Oh, right, I forgot about that particular abomination of language design.

Why can't we (also) accept comma-delimited strings? That's what we do everywhere else.

[alexreinking]

I suppose we could silently turn , into ; for just that variable if that would always be correct. It needs to be processed internally as a CMake list and the TARGETS argument to add_halide_library is naturally a CMake list, too. So this was for consistency / code simplicity.

[abadams]

I would be in favor of just unpacking comma-separated targets into lists as if the commas were semicolons