Deprecate sdl2-config and AM_PATH_SDL2

[smcv]

If we deprecate these in SDL 2.x, then developers of dependent projects will have had some advance warning before they're removed in SDL 3 (which I think should only install ${libdir}/pkgconfig/sdl3.pc and ${libdir}/cmake/SDL3/*.cmake, with no replacement for ${bindir}/sdl2-config or ${datadir}/aclocal/sdl2.m4).

• sdl2-config.in: Deprecate sdl2-config

  Library-specific foo-config scripts duplicate very similar logic across
  various different projects, and tend to break cross-compiling, multilib
  (gcc -m32), Debian/Ubuntu multiarch and so on by only being able to have
  one sdl2-config at a time as the first one in the PATH.

  The direct replacement is pkg-config(1) or a compatible reimplementation
  like pkgconf(1), which relies on each library installing declarative
  metadata, like SDL's sdl2.pc (available since at least 2.0.0) and
  centralizes the logic into the pkg-config/pkgconf tool.

  Most uses of sdl2-config --foo can be replaced by something similar
  to ${PKG_CONFIG:-pkg-config} --foo sdl2. Instead of adding a custom
  sdl2-config to the PATH or using its --prefix or --exec-prefix options,
  users of a custom installation prefix can use any of pkg-config's
  non-SDL-specific ways to influence the result, for example setting
  PKG_CONFIG_PATH, PKG_CONFIG_SYSROOT_DIR or PKG_CONFIG_LIBDIR environment
  variables, or setting the PKG_CONFIG environment variable to point to
  a wrapper script.

  For Autotools specifically, the replacement for AM_PATH_SDL2 (which
  will be officially deprecated in a subsequent commit) is
  PKG_CHECK_MODULES.

  CMake has its own semi-declarative mechanism for dependency discovery,
  "config packages", and the SDL build already installs a config
  package. There's a good example of using a config package to discover
  SDL in cmake/test/.

  Meson natively supports pkg-config, and already uses it in preference
  to sdl2-config.

  Other build systems can run pkg-config instead of sdl2-config,
  preferably checking the PKG_CONFIG environment variable first.
  https://github.com/ioquake/ioq3 is a good example of a project doing
  this correctly.

  Helps: Deprecate sdl2-config and AM_PATH_SDL2, at least on Unix #6140, On Windows, sdl2-config should be written in something other than bash #3516

• sdl2.m4: Deprecate AM_PATH_SDL2 in favour of PKG_CHECK_MODULES

  AM_PATH_SDL2 doesn't add much beyond PKG_CHECK_MODULES, and having a
  special m4 macro for every library that you might depend on scales
  poorly.

  The macro does add special support for macOS frameworks, but that feature
  was broken for around 6 years without anyone noticing (sdl2.m4 support for macOS frameworks probably broken #6141), and is
  likely to be only rarely useful according to comments on sdl2.m4 support for macOS frameworks probably broken #6141.

  Resolves: Deprecate sdl2-config and AM_PATH_SDL2, at least on Unix #6140

Existing Issue(s)

#6140

[sezero]

Should we revert this one? C.f.: #6494

[slouken]

Let's leave this as-is for now, and revisit with SDL3.