Check for sdl-config before calling it

Check whether sdl-config is available before calling it, otherwise
./configure triggers a warning:
    ./configure: 957: sdl-config: not found

If neither the .pc file not sdl-config are present, disable SDL support.
Signed-off-by: NLoïc Minier <lool@dooz.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

configure

@@ -997,9 +997,14 @@ fi
 if $pkgconfig sdl --modversion >/dev/null 2>&1; then
   sdlconfig="$pkgconfig sdl"
   _sdlversion=`$sdlconfig --modversion 2>/dev/null | sed 's/[^0-9]//g'`
-else
+elif which sdl-config >/dev/null 2>&1; then
   sdlconfig='sdl-config'
   _sdlversion=`$sdlconfig --version | sed 's/[^0-9]//g'`
+else
+  if test "$sdl" = "yes" ; then
+    feature_not_found "sdl"
+  fi
+  sdl=no
 fi
 
 sdl_too_old=no