configure: avoid basename usage message

basename prints a missing-argument error when sdlconfig is empty
and we're cross-compiling.
Signed-off-by: NScott Wood <scottwood@freescale.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -1233,7 +1233,7 @@ else
   fi
   sdl=no
 fi
-if test -n "$cross_prefix" && test "`basename $sdlconfig`" = sdl-config; then
+if test -n "$cross_prefix" && test "$(basename "$sdlconfig")" = sdl-config; then
   echo warning: using "\"$sdlconfig\"" to detect cross-compiled sdl >&2
 fi