BUILD: makefile: support SYSTEMD_LDFLAGS

The LDFLAGS appended by USE_SYSTEMD can now be overridden using
SYSTEMD_LDFLAGS.
diff --git a/Makefile b/Makefile
index 8e72364..0cafdad 100644
--- a/Makefile
+++ b/Makefile
@@ -713,7 +713,7 @@
 endif
 
 ifneq ($(USE_SYSTEMD),)
-  OPTIONS_LDFLAGS += -lsystemd
+  SYSTEMD_LDFLAGS = -lsystemd
 endif
 
 ifneq ($(USE_PCRE)$(USE_STATIC_PCRE)$(USE_PCRE_JIT),)