Provide our own psqlscan.l->psqlscan.l rule.

As postgres's generic .l -> .c Makefile rule uses ifdef - which is
evaluated early, not during rule evaluation - we have to override the
rule, in addition to the detection of FLEX in the previous commit.

Fixes: #439

src/bin/csql/Makefile

@@ -36,7 +36,9 @@ override CPPFLAGS := -I$(client_includes) -I$(libpq_srcdir) -I$(citus_abs_top_sr
 # psqlscan is compiled as part of mainloop
 mainloop.o: psqlscan.c
 psqlscan.c: FLEXFLAGS = -Cfe -p -p
-psqlscan.c: FLEX_NO_BACKUP=yes
+
+psqlscan.c: psqlscan.l
+	$(FLEX) $(FLEXFLAGS) -o'$@' $<
 
 clean: csql-clean
 csql-clean: