configure: enable sdl if sdl2 is enabled

sdl is supposed to be an alias, so it should have the same value.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

configure

@@ -5894,7 +5894,7 @@ if ! disabled sdl2; then
         sdl2_libs="$sdl2_libs -mconsole"
     fi
 fi
-enabled sdl2 && add_cflags $sdl2_cflags && add_extralibs $sdl2_libs
+enabled sdl2 && enable sdl && add_cflags $sdl2_cflags && add_extralibs $sdl2_libs
 
 disabled securetransport || { check_func SecIdentityCreate "-Wl,-framework,CoreFoundation -Wl,-framework,Security" &&
     check_lib2 "Security/SecureTransport.h Security/Security.h" "SSLCreateContext SecItemImport" "-Wl,-framework,CoreFoundation -Wl,-framework,Security" &&