Fix portability issue in Makefile

The == test operator is not defined by POSIX

tests/Makefile

@@ -433,7 +433,7 @@ ifndef WINDOWS
 		RESULT=`$(CHECK_PRELOAD) ./$${i} | grep -v 'PASS$$' | grep -v -- '----' | grep -v '^$$'`;	\
 		PASSED=`echo $$RESULT |grep PASSED`; 								\
 		echo "   $$RESULT";													\
-		if [ "$$PASSED" = "" ];											\
+		if [ "x$$PASSED" = "x" ];											\
 		then																\
 			echo "**** Failed ***************";								\
 			RETURN=1;														\