POSIX fix


git-svn-id: svn://svn.rockbox.org/rockbox/trunk@11446 a1c6a512-1295-4272-9138-f99709370657
diff --git a/manual/Makefile b/manual/Makefile
index 233ab37..17964fa 100644
--- a/manual/Makefile
+++ b/manual/Makefile
@@ -28,6 +28,6 @@
 	$(MAKE) -C $(OBJDIR) htmlzip
 
 clean:
-	@if [ "$(OBJDIR)" == "" ]; then echo Run make in you build directory!; false; fi
+	@if [ "$(OBJDIR)" = "" ]; then echo Run make in you build directory!; false; fi
 	@rm -rf $(OBJDIR)/manual $(OBJDIR)/*.pdf
 	@rm -rf $(OBJJDIR)/html $(OBJDIR)/txt