libubacktrace: fix config-symbol handling

Config symbals that are not set are empty, not 'n'.

libubacktrace/Makefile.in

@@ -45,7 +45,7 @@ lib-so-$(UCLIBC_HAS_BACKTRACE) += $(top_builddir)lib/libubacktrace.so
 
 objclean-y += CLEAN_libubacktrace
 
-ifeq ($(DOMULTI),n)
+ifeq ($(DOMULTI),)
 ifeq ($(DOPIC),y)
 $(top_builddir)lib/libubacktrace.so: $(top_builddir)lib/libubacktrace.a $(libdl.depend)
 else