configure.ac : Fix --disable-sse option.

Based on a patch from lvqcl <lvqcl.mail@gmail.com>.
diff --git a/configure.ac b/configure.ac
index 7d58f02..f74bb1d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -416,7 +416,7 @@
 		XIPH_ADD_CFLAGS([-fgnu89-inline])
 		fi
 
-	if test "x$asm_optimisation" = "xyes" ; then
+	if test "x$asm_optimisation$sse_os" = "xyesyes" ; then
 		XIPH_ADD_CFLAGS([-msse2])
 		fi