Remove sdl3.m4 and sdl3-config, and document pkg-config as the recommended replacement

Fixes https://github.com/libsdl-org/SDL/issues/6639

cmake/test/test_sdlconfig.sh

@@ -1,51 +0,0 @@
-#!/bin/sh
-
-if test "x$CC" = "x"; then
-    CC=cc
-fi
-
-machine="$($CC -dumpmachine)"
-case "$machine" in
-    *mingw* )
-        EXEPREFIX=""
-        EXESUFFIX=".exe"
-        ;;
-    *android* )
-        EXEPREFIX="lib"
-        EXESUFFIX=".so"
-        EXTRA_LDFLAGS="$EXTRA_LDFLAGS -shared"
-        ;;
-    * )
-        EXEPREFIX=""
-        EXESUFFIX=""
-        ;;
-esac
-
-set -e
-
-# Get the canonical path of the folder containing this script
-testdir=$(cd -P -- "$(dirname -- "$0")" && printf '%s\n' "$(pwd -P)")
-CFLAGS="$( sdl3-config --cflags )"
-LDFLAGS="$( sdl3-config --libs )"
-STATIC_LDFLAGS="$( sdl3-config --static-libs )"
-
-compile_cmd="$CC -c "$testdir/main_gui.c" -o main_gui_sdlconfig.c.o $CFLAGS $EXTRA_CFLAGS"
-link_cmd="$CC main_gui_sdlconfig.c.o -o ${EXEPREFIX}main_gui_sdlconfig${EXESUFFIX} $LDFLAGS $EXTRA_LDFLAGS"
-static_link_cmd="$CC main_gui_sdlconfig.c.o -o ${EXEPREFIX}main_gui_sdlconfig_static${EXESUFFIX} $STATIC_LDFLAGS $EXTRA_LDFLAGS"
-
-echo "-- CC:                $CC"
-echo "-- CFLAGS:            $CFLAGS"
-echo "-- EXTRA_CFLAGS:      $EXTRA_CFLAGS"
-echo "-- LDFLAGS:           $LDFLAGS"
-echo "-- STATIC_LDFLAGS:    $STATIC_LDFLAGS"
-echo "-- EXTRA_LDFLAGS:     $EXTRA_LDFLAGS"
-
-echo "-- COMPILE:       $compile_cmd"
-echo "-- LINK:          $link_cmd"
-echo "-- STATIC_LINK:   $static_link_cmd"
-
-set -x
-
-$compile_cmd
-$link_cmd
-$static_link_cmd