configure: fix sdl detection using sdl2-config

If SDL2 is requested but pkg-config doesn't have a module for it
configure should fallback to use sdl*-config, but wasn't able to
because and old variable (from SDL) was being used by mistake.

Correct the variable name and complete other related changes so
there are no more references to the old SDL.

Fixes: 0015ca5c ("ui: remove support for SDL1.2 in favour of SDL2")
Signed-off-by: NCarlo Marcelo Arenas Belón <carenas@gmail.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-Id: <20190710225528.409-1-carenas@gmail.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

configure

@@ -3022,15 +3022,15 @@ fi
 ##########################################
 # SDL probe
 
-# Look for sdl configuration program (pkg-config or sdl-config).  Try
-# sdl-config even without cross prefix, and favour pkg-config over sdl-config.
+# Look for sdl configuration program (pkg-config or sdl2-config).  Try
+# sdl2-config even without cross prefix, and favour pkg-config over sdl2-config.
 
 sdl_probe ()
 {
   if $pkg_config sdl2 --exists; then
     sdlconfig="$pkg_config sdl2"
     sdlversion=$($sdlconfig --modversion 2>/dev/null)
-  elif has ${sdl_config}; then
+  elif has "$sdl2_config"; then
     sdlconfig="$sdl2_config"
     sdlversion=$($sdlconfig --version)
   else
@@ -3041,7 +3041,7 @@ sdl_probe ()
     # no need to do the rest
     return
   fi
-  if test -n "$cross_prefix" && test "$(basename "$sdlconfig")" = sdl-config; then
+  if test -n "$cross_prefix" && test "$(basename "$sdlconfig")" = sdl2-config; then
     echo warning: using "\"$sdlconfig\"" to detect cross-compiled sdl >&2
   fi
 
@@ -8023,7 +8023,6 @@ preserve_env PKG_CONFIG
 preserve_env PKG_CONFIG_LIBDIR
 preserve_env PKG_CONFIG_PATH
 preserve_env PYTHON
-preserve_env SDL_CONFIG
 preserve_env SDL2_CONFIG
 preserve_env SMBD
 preserve_env STRIP