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: #6140, #3516
Signed-off-by: Simon McVittie <smcv@collabora.com>

sdl2-config.in

@@ -19,6 +19,11 @@ if test $# -eq 0; then
       exit 1
 fi
 
+echo "sdl2-config: This script is deprecated" >&2
+echo "sdl2-config: In Autotools builds, use PKG_CHECK_MODULES([SDL], [sdl2 >= 2.x.y])" >&2
+echo "sdl2-config: In CMake builds, use find_package(SDL2 CONFIG)" >&2
+echo "sdl2-config: In other build systems, look for 'sdl2' with pkg-config(1) or pkgconf(1)" >&2
+
 while test $# -gt 0; do
   case "$1" in
   -*=*) optarg=`echo "$1" | sed 's/[-_a-zA-Z0-9]*=//'` ;;