Fix the with_SDL test.

Author: blast007

configure.ac

@@ -557,7 +557,7 @@ case $host_os in
     darwin*)
 	;;
     *)
-      if "x$with_SDL" != xyes; then
+      if test x$with_SDL != xyes; then
 	CONF_CPPFLAGS="$CONF_CPPFLAGS $X_CFLAGS"
 	if test x$no_x = x; then
 	    GLIBS="$X_PRE_LIBS -lX11 $EXTRA_LIBS $GLIBS"