Improve processing of SQLITE_NULL

null-atom can be configured on configure or make stage. For example
./configure --enable-null-atom=undefined && make

priv/Makefile.in

@@ -4,7 +4,7 @@ GCC=gcc
 LDFLAGS=-shared -L/usr/lib -lsqlite3 -undefined -fPIC -lerl_interface -L$(ERL_INTERFACE)/lib -lei
 SRCS=sqlite3_drv.c sqlite3_drv.h
 OUTPUT=sqlite3_drv.so
-CFLAGS=-o ../ebin/${OUTPUT} -I$(ERL_ROOT)/usr/include/ -I$(ERL_INTERFACE)/include/ 
+CFLAGS=-o ../ebin/${OUTPUT} -I$(ERL_ROOT)/usr/include/ -I$(ERL_INTERFACE)/include/ -DNULL_ATOM=@NULL_ATOM@
 
 all: ../ebin/${OUTPUT}