BUILD: makefile: support BACKTRACE_LDFLAGS

The LDFLAGS appended by USE_BACKTRACE can now be overridden using
BACKTRACE_LDFLAGS.
diff --git a/Makefile b/Makefile
index 71bd1d5..8e72364 100644
--- a/Makefile
+++ b/Makefile
@@ -552,7 +552,7 @@
 endif
 
 ifneq ($(USE_BACKTRACE),)
-  OPTIONS_LDFLAGS += -Wl,$(if $(EXPORT_SYMBOL),$(EXPORT_SYMBOL),--export-dynamic)
+  BACKTRACE_LDFLAGS = -Wl,$(if $(EXPORT_SYMBOL),$(EXPORT_SYMBOL),--export-dynamic)
 endif
 
 ifneq ($(USE_CPU_AFFINITY),)