make target completion

[Konfekt]

How about make target completion as in https://github.com/mg979/tasks.vim/blob/559175d47ea8fd7f48916882874484e22c55c557/autoload/async/make.vim for the make compiler in

vim-dispatch/autoload/dispatch.vim

Line 732 in a2ff28a

let fn = matchstr(line, '\C-complete=\zscustom\%(list\)\=,\%(s:\)\@!\S\+')

?