BUILD: makefile: support LIBCRYPT_LDFLAGS

The LDFLAGS appended by USE_LIBCRYPT can now be overridden using
LIBCRYPT_LDFLAGS.
diff --git a/Makefile b/Makefile
index f45cc44..a523553 100644
--- a/Makefile
+++ b/Makefile
@@ -512,7 +512,7 @@
 ifneq ($(USE_LIBCRYPT),)
   ifneq ($(TARGET),openbsd)
     ifneq ($(TARGET),osx)
-      OPTIONS_LDFLAGS += -lcrypt
+      LIBCRYPT_LDFLAGS = -lcrypt
     endif
   endif
 endif