Take advantage of new Makefile in appveyor.yml

See merge request !1561

Also changed -Wno-error=implicit-fallthrough to
-Wno-implicit-fallthrough. For some reason Appveyor's
version of GCC is triggering these warnings despite the
comments, so just shut it up.

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.

Some issues with the Makefile

See merge request !1553

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use an outdated executable if it is one of those untracked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.

Fix console text bleeding

See merge request STJr/SRB2!1434

Add badge to readme showing latest stable release

See merge request STJr/SRB2!1510

Fix some compiler warnings

See merge request STJr/SRB2!1504

The year is 2021

See merge request STJr/SRB2!1500

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