Makefile: use `wildcard` instead of `shell ls`
Makefile: use `wildcard` instead of `shell ls`

I introduced `shell ls` in 743d6352430b2f7b49a0e6e83bb0f993bf3c364b and
0da810b5e11974c0f33a51654ec62e6da0aa3475 because I didn't know better,
even through `wildcard` was used right there in the same Makefile.

Now it backfired: a user on the mailing list ran into problems[1]
building Newsboat because they had their own version of `ls` that
returned different results for different directories.

1. https://groups.google.com/forum/#!topic/newsboat/mbKg0JmgXdc

newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat