undocumented dependency on perl

[marler8997]

It looks like line 57 of src/Makefile on version 0.6 calls perl to do a substitution in src/luaconf.h. However, the README.md doesn't document any dependency on perl.

I found this by using nix to build howl in a "pure" environment with:

# fails with perl: Command not found
nix-shell --pure -p wget pkgconfig gtk3 --run "make"

# adding perl to the list of packages works
nix-shell --pure -p wget pkgconfig gtk3 perl --run "make"