BUILD: makefile: support RT_LDFLAGS

The LDFLAGS appended by USE_RT can now be overridden using
RT_LDFLAGS.
diff --git a/Makefile b/Makefile
index a523553..63c7f78 100644
--- a/Makefile
+++ b/Makefile
@@ -544,7 +544,7 @@
 endif
 
 ifneq ($(USE_RT),)
-  OPTIONS_LDFLAGS += -lrt
+  RT_LDFLAGS = -lrt
 endif
 
 ifneq ($(USE_THREAD),)