configure.ac: set sdl_version after sdl checked

For actual sdl check, the emptiness of sdl_version is evaluated. But later
it is useful to have it =0 for numeric comparisons (2c577ac) so set it
in after sdl check. The depending modules can use $sdl variable later.

Author: MartinPulec

configure.ac

@@ -1242,7 +1242,7 @@ fi
 # SDL1/2/3 Stuff
 # ------------------------------------------------------------------------------
 sdl=no
-sdl_version=0
+sdl_version=
 sdl_version_str=
 sdl_ver_suffix=
 AC_ARG_ENABLE(sdl,
@@ -1308,6 +1308,8 @@ if test "$sdl_req" != no; then
         fi
 fi
 
+sdl_version=${sdl_version:-0}
+
 ENSURE_FEATURE_PRESENT([$sdl_req], [$sdl], [SDL requested but found any version])
 
 # ------------------------------------------------------------------------------