build/clean leaves files

[gdt]

After building 2.53.0alpha2 on NetBSD 9, and then cleaning, a few files remain, as shown by git status --ignored:

Ignored files:
  (use "git add -f <file>..." to include in what will be committed)
        src/lwt/generic/lwt_unix_impl.o
        src/lwt/lwt.o
        src/lwt/lwt_unix.o
        src/lwt/lwt_util.o
        src/lwt/pqueue.o
        src/system/generic/system_impl.o
        src/system/system_generic.o
        src/ubase/depend

[gdt]

The bug appears to be constructions not specified by POSIX shell: *.{o,obj}, and it therefore needs a bash exorcism.