Use XPDEV_CFLAGS when building.

Fixes at least an snprintf() issue on macOS, and likely many other
things.  It's actually surprising this builds without it to be honest.

src/smblib/GNUmakefile

@@ -17,7 +17,7 @@ SRC_ROOT = ..
 # Cross platform/compiler definitions
 include $(SRC_ROOT)/build/Common.gmake	# defines clean and output directory rules
 
-CFLAGS += -I$(XPDEV_SRC) $(ENCODE_CFLAGS) $(HASH_CFLAGS)
+CFLAGS += -I$(XPDEV_SRC) $(ENCODE_CFLAGS) $(HASH_CFLAGS) $(XPDEV_CFLAGS)
 
 
 # SMBLIB Library Link Rule