build: isolate sdl-to-sdl2 aliasing

This simplifies incoming SDL related changes by removing potential
mismatching states of sdl and sdl2 variables. Since a component can have
all kind of states (such as unset, enabled, disabled or requested),
keeping these variables in sync manually in random places is not robust.

configure

@@ -1488,7 +1488,6 @@ EXTERNAL_AUTODETECT_LIBRARY_LIST="
     libxcb_xfixes
     lzma
     schannel
-    sdl
     sdl2
     securetransport
     xlib
@@ -3524,6 +3523,9 @@ for opt do
             action=${opt%%-random=*}
             do_random ${action#--} $optval
         ;;
+        --enable-sdl)
+            enable sdl2
+        ;;
         --enable-*=*|--disable-*=*)
             eval $(echo "${opt%%=*}" | sed 's/--/action=/;s/-/ thing=/')
             is_in "${thing}s" $COMPONENT_LIST || die_unknown "$opt"
@@ -6025,7 +6027,6 @@ if enabled gcrypt; then
     fi
 fi
 
-disabled sdl && disable sdl2
 if ! disabled sdl2; then
     SDL2_CONFIG="${cross_prefix}sdl2-config"
     if check_pkg_config sdl2 SDL_events.h SDL_PollEvent; then
@@ -6045,7 +6046,7 @@ if ! disabled sdl2; then
         sdl2_extralibs="$sdl2_extralibs -mconsole"
     fi
 fi
-enabled sdl2 && enable sdl && add_cflags $sdl2_cflags && add_extralibs $sdl2_extralibs
+enabled sdl2 && add_cflags $sdl2_cflags && add_extralibs $sdl2_extralibs
 
 if enabled decklink; then
     case $target_os in