use pkg-config for sdl whenever available

Together with the first patch this enables using the prefixed
pkg-config, thus picking up the correct flags for SDL.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -990,18 +990,24 @@ fi
 ##########################################
 # SDL probe
 
-sdl_too_old=no
+if $pkgconfig sdl --modversion >/dev/null 2>&1; then
+  sdlconfig="$pkgconfig sdl"
+  _sdlversion=`$sdlconfig --modversion 2>/dev/null | sed 's/[^0-9]//g'`
+else
+  sdlconfig='sdl-config'
+  _sdlversion=`$sdlconfig --version | sed 's/[^0-9]//g'`
+fi
 
+sdl_too_old=no
 if test "$sdl" != "no" ; then
   cat > $TMPC << EOF
 #include <SDL.h>
 #undef main /* We don't want SDL to override our main() */
 int main( void ) { return SDL_Init (SDL_INIT_VIDEO); }
 EOF
-  sdl_cflags=`sdl-config --cflags 2> /dev/null`
-  sdl_libs=`sdl-config --libs 2> /dev/null`
+  sdl_cflags=`$sdlconfig --cflags 2> /dev/null`
+  sdl_libs=`$sdlconfig --libs 2> /dev/null`
   if compile_prog "$sdl_cflags" "$sdl_libs" ; then
-    _sdlversion=`sdl-config --version | sed 's/[^0-9]//g'`
     if test "$_sdlversion" -lt 121 ; then
       sdl_too_old=yes
     else