Make the Makefile 'all' target phony and default

[mathisto]

I noticed in the README, we are encouraging users to include the docs by default.

I also noticed the makefile had an empty all target. GNU make, when invoked without an explicit target, will snag the first one it finds.

This PR simply adds the default install and doc targets the default. I updated the README to reflect this as well.

[Euro20179]

Someone else said it was a bad idea to do this exact thing, I think they were just saying it's non-standard to do this, but I'm not sure.

[mathisto]

Ahh, I see.

Gee willikers! Boy do I not like make.

So, I dug in a bit on this, and I think this is referencing 'standard targets for users'. It should be noted that this is not a POSIX requirement. The all target has no special meaning to make whatsoever. That being said, GNU manual does recommend that programs use it, and additionally admonishes it NOT include the docs.

So as to keep the all directive intact, useless as it is, I added the .DEFAULT_GOAL directive which does have special significance to make. This directive was added in v3.80. Version 3.80 just had its 20th birthday on the 3rd, so we should be good for portability, methinks.

Let me know what you think.

[Euro20179]

Version 3.80 just had its 20th birthday on the 3rd, so we should be good for portability, methinks.

Honestly you would think it would be portable, but I tried with the arch package bmake (bsd make) and it does nothing.

However, it also doesn't break bmake so I think it's fine, you would just have to do it the old way (bmake install doc), so you should probably keep the readme the same.

Thanks for doing the research on make btw.

[mathisto]

OOOF. You know, I've never even used BSD and it makes feel downright silly sometimes.
Ok, I'll pull the readme changes. Thanks for working with me on this. ✋

[mathisto]