Some key points for programmers:
- Source code files are mostly listed in a 'Sourcefile'.
  So you no longer directly edit the object list. There
  can be multiple Sourcefiles and they can even live in
  subdirectories--the directory name will be prepended to
  every filename in the list. Of course, the Makefile
  still needs to be edited to read from each Sourcefile.
- Different rules are no longer required for source code
  files that live in subdirectories (such as sdl/ or
  hardware/). Subdirectories Just Work so go ham!

In addition to those points, another important change is
that the bin directory is no longer divided into platform
subdirectories (Linux64, Mingw, etc). Executables now go
directly into bin. If you use DEBUGMODE or target 64-bit,
then subdirectories for 'debug' and '64' will be made
though.

Oh by the way, I don't think make clean actually removed
files before on Windows. It should now. I also fixed as
many little inconsistencies like that as I noticed.

And now just an overview of the technical aspects that
shouldn't affect anyone who doesn't REALLY care about the
Makefile...

objs and dep directories have been moved to a make
directory. Makefile.cfg and its variants have been moved
out of their various subdirectories to src/Makefile.d
make distclean removes the bin and make directories
entirely, but make clean and cleandep still only affect
the current build target.

When I say automation, I mean that a lot of copy pasting
in the Makefile has been reduced.

BRUH MOMENT

Nice UDB config there, SRB2.

See merge request !1354

Dependency files

See merge request !1407

This makes it print nothing to stdout.

Also fixed some irregularities.

This removes Makefile.depends. Instead, '.d' files are included from the 'dep'
directory. This speeds up building because dependencies for every file don't
need to be regenerated if only one changes.

As a bonus, dependencies also won't be generated if only clean type targets
are going to be run. Also added a 'distclean' target, which cleans both
objects and dependency files.

Remove Direct Draw from Makefile

See merge request STJr/SRB2!1263

Echo filenames when compiling without ECHO=1

See merge request STJr/SRB2!1260

MINGW/MINGW64 implies SDL.

Remove direct definition for D__USE_MINGW_ANSI_STDIO, as the latest standards consider this to be deprecated.

Cmake: fullscreen fix

See merge request STJr/SRB2!1226

Fix compiling using cmake if internal libs is used

See merge request STJr/SRB2!1225

Use SSE3 in 32-bit x86 binaries

See merge request STJr/SRB2!1213

All because of a typo

Add multithreading option for CMake

See merge request STJr/SRB2!1204

Fix gzip failing to compress SRB2's objdump on non-Windows.

See merge request STJr/SRB2!1191

2.2.8 prep

See merge request STJr/SRB2!1170