handle absence of wget gracefully about erlang.mk

[essen]

The issue will be solved not by trying every HTTP client possible, but by using Guile bundled with GNU Make 4 and only running wget on fallback. So it will be the same code for all platforms and will just keep compatibility with what we had so far until we phase out GNU Make 3.

[talklittle]

@essen Using a bundled Guile HTTP client is reasonable in the future. But from what I see, the latest OSX still ships with GNU Make 3.81, so that solution still wouldn't support a fresh OSX installation.

[essen]

I'm not in a hurry.

[Licenser]

I think that wget+curl covers 99% of all people (outside windows) so it's not about trying every possible client just not forcing users into exactly one.

OS X is a quite prominent system for developers so supporting it does not seem that bad of a idea.

[essen]

I got told version 4 is available in homebrew, that's good enough support for me.

The dependencies for erlang.mk will become over time gnu-make4, erlang and openssl (yes, openssl is already an erlang dep on some platforms). It will be the same for all platforms. Some of them will simply have an easier initial setup than others. All platforms have some kind of initial setup which will all be documented (some with other people's help of course). That initial setup will be easier for everyone, including OSX, and will make erlang.mk more stable because it will be the same code for all platforms.

Note that make isn't part of the fresh OSX install either. So it's not like anyone can just use erlang.mk without any initial setup. The only difference is that instead of that 8 years old version you need this more recent version, and the README will explain where it can be obtained, and trying to use erlang.mk without guile support will also trigger a warning.

[talklittle]

@essen Thank you for expanding on the reasons behind your decision, and giving us a glimpse of your future plans for the project. It makes it a lot easier to accept your decision, and see that you are being perfectly reasonable in limiting external dependencies.

I'll at least open a new Pull Request that complains if there's no wget in the $PATH. Currently on OSX, without wget, running make fails with a cryptic error about the missing .erlang.mk.packages.v1 file. No indication that a missing wget is the cause for failure.

EDIT: Sorry, it does also complain about missing wget. Didn't notice last time. /bin/sh: wget: command not found

[essen]

Probably a good idea to make the fallback use Erlang directly.

[essen]

If anyone wants to add a fallback that uses Erlang to download the file, please do!

[essen]

We now have a fallback that uses "erl" for downloading the file. Enjoy!