Simplify application Makefile boilerplate setup

[sampsyo]

Currently, to set up a new application, you have to create a Makefile with some boilerplate in it:

TARGET := foo
APP_MK := ../app.mk
include $(APP_MK)

It would be nice to streamline this process:

• [x] Don't require a TARGET definition; just use a dumb default name like "program" by default.
• [ ] Avoid requiring a reference to app.mk. It would be great if an application didn't need to know where ACCEPT was located so the application could be relocatable.
• [ ] Perhaps applications shouldn't need to have a Makefile at all if they don't want to customize it—perhaps the driver can automatically create/append to the Makefile before running anything.

[sampsyo]

Once this is done, we should also document the way Makefiles work (and app.mk in particular) so people have a chance at understanding how to do more advanced things with their build workflows.

[sampsyo]

A marginally nicer alternative to what we have now would be to use make's include paths. The required line could be just include app.mk with the right path set up with .INCLUDE_DIRS. We could then extend he path with a flag.

[sampsyo]

.INCLUDE_DIRS is apparently not assignable. Giving up on this.