Add SDL 3 support to the Makefile.

This is hidden behind WITH_SDL3, which is disabled by default.

Additionally rename the SDL sources files in the client to represent the
SDL major version they are supporting. They will be forked for SDL 3.
That isn't optimal, because it forces us to support two variants of the
same code. However the changes between SDL 2 and 3 are too big to work
with #ifdef and something like function pointer magic is even more
confusing.

Makefile

@@ -53,6 +53,9 @@ WITH_OPENAL:=yes
 # or libopenal.so. Not supported on Windows.
 WITH_RPATH:=yes
 
+# Builds with SDL 3 instead of SDL 2.
+WITH_SDL3:=no
+
 # Enable systemwide installation of game assets.
 WITH_SYSTEMWIDE:=no
 
@@ -276,7 +279,12 @@ endif
 # ----------
 
 # Extra CFLAGS for SDL.
+ifeq ($(WITH_SDL3),yes)
+SDLCFLAGS := $(shell pkgconf --cflags sdl3)
+SDLCFLAGS += -DUSE_SDL3
+else
 SDLCFLAGS := $(shell sdl2-config --cflags)
+endif
 
 # ----------
 
@@ -353,11 +361,19 @@ endif
 # ----------
 
 # Extra LDFLAGS for SDL
+ifeq ($(WITH_SDL3),yes)
+ifeq ($(YQ2_OSTYPE), Darwin)
+SDLLDFLAGS := -lSDL3
+else
+SDLLDFLAGS := $(shell pkgconf --libs sdl3)
+endif
+else
 ifeq ($(YQ2_OSTYPE), Darwin)
 SDLLDFLAGS := -lSDL2
-else # not Darwin
+else
 SDLLDFLAGS := $(shell sdl2-config --libs)
-endif # Darwin
+endif
+endif
 
 # The renderer libs don't need libSDL2main, libmingw32 or -mwindows.
 ifeq ($(YQ2_OSTYPE), Windows)
@@ -394,6 +410,7 @@ config:
 	@echo "WITH_CURL = $(WITH_CURL)"
 	@echo "WITH_OPENAL = $(WITH_OPENAL)"
 	@echo "WITH_RPATH = $(WITH_RPATH)"
+	@echo "WITH_SDL3 = $(WITH_SDL3)"
 	@echo "WITH_SYSTEMWIDE = $(WITH_SYSTEMWIDE)"
 	@echo "WITH_SYSTEMDIR = $(WITH_SYSTEMDIR)"
 	@echo "============================"
@@ -852,17 +869,14 @@ CLIENT_OBJS_ := \
 	src/client/cl_view.o \
 	src/client/curl/download.o \
 	src/client/curl/qcurl.o \
-	src/client/input/sdl.o \
 	src/client/menu/menu.o \
 	src/client/menu/qmenu.o \
 	src/client/menu/videomenu.o \
-	src/client/sound/sdl.o \
 	src/client/sound/ogg.o \
 	src/client/sound/openal.o \
 	src/client/sound/qal.o \
 	src/client/sound/sound.o \
 	src/client/sound/wave.o \
-	src/client/vid/glimp_sdl.o \
 	src/client/vid/vid.o \
 	src/common/argproc.o \
 	src/common/clientserver.o \
@@ -898,6 +912,18 @@ CLIENT_OBJS_ := \
 	src/server/sv_user.o \
 	src/server/sv_world.o
 
+ifeq ($(WITH_SDL3),yes)
+CLIENT_OBJS_ += \
+	src/client/input/sdl3.o \
+	src/client/sound/sdl3.o \
+	src/client/vid/glimp_sdl3.o
+else
+CLIENT_OBJS_ += \
+	src/client/input/sdl2.o \
+	src/client/sound/sdl2.o \
+	src/client/vid/glimp_sdl2.o
+endif
+
 ifeq ($(YQ2_OSTYPE), Windows)
 CLIENT_OBJS_ += \
 	src/backends/windows/main.o \