Previously took only the filename, so the directory
component was stripped. This broke dependencies for
basically every file.

According to this answer--
https://stackoverflow.com/a/45952425
--Make will always prefer a unix shell, even on Windows,
if one can be found in Path. So we can't check PATH to
determine if it's a Windows shell... this is just too much
bother.

If a variable is defined as in 'make CC=gcc-10', then that
definition overrides anything other definition in the
Makefile.

After a checkout from before revision, old directories
such as bin/Linux64 only remain if untracked files exist
within. This may be confusing to the user. They may even
use an outdated executable if it is one of those untracked
files.

This is for MSYS2, which requires unix shell commands.

File function is not supported < Make 4.2.

With added textual contrast.

NOOBJDUMP=1 and NOUPX=1 have been removed. Make 'dump'
target to disassemble. UPX=upx to compress executable.

Setting UPX used to cause it to fail. That has been fixed.

Some of these integers exceed the precision of float. In
that case the number is rounded. The rounding shouldn't
matter too much anyway, so just shut the compiler up.

- target_sources from correct directory
- enable_language must be used in add_executable directory

This establishes (near) parity of source code file lists
between the Makefile and CMakeLists.txt

To make that change I messed around CMakeLists.txt a bit.
It now uses target_sources and target_compile_definitions.
I also removed some MSVC stuff since we don't actually
care about MSVC--it made things easier.

CMake minimum version 3.0 -> 3.13 for target_sources.

- Quotes were not removed by ECHO.
- DEL would print an error on nonexistent file.
- MD would do this plus return a nonzero exit code.

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.

See 7df6a309 and 83a87042. I didn't realize that light
userdata's metatable is shared--like numbers or strings.
So it cannot be paired with a metatable.

I also made a few minor tweaks to Lua cvars, other than
accounting for the double pointer in the userdata.

This reverts commit d4c08a84, reversing
changes made to e100f21d.

This reverts commit c51c4787.