make clean didn't clean up files generated from *.(y|l)

Hello,

I got the build error below. It concerns RESERVE_WAL is not defined,
however, it should not be a problem to be oversight for a long time.

I tried to build the latest master branch just after the switch from
REL9_5_STABLE and "make clean", however, repl_gram.c was not cleaned
up correctly. So, my problem is that repl_gram.l was the latest version,
but compiler saw the repl_gram.c generated based on the v9.5 source.

I could see the similar problems at:
src/backend/replication/repl_gram.c
src/interfaces/ecpg/preproc/pgc.c
src/bin/pgbench/exprparse.c
src/bin/pgbench/exprscan.c
src/pl/plpgsql/src/pl_gram.c

(*) At least, these files raised a build error.

Probably, we have to add explicit cleanup of these auto-generated files
on Makefiles.

Thanks,

----------------
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -O2 -O0 -g -I. -I. -I../../../src/include -D_GNU_SOURCE -c -o repl_gram.o repl_gram.c
In file included from repl_gram.y:320:0:
repl_scanner.l: In function 'replication_yylex':
repl_scanner.l:98:10: error: 'K_RESERVE_WAL' undeclared (first use in this function)
RESERVE_WAL { return K_RESERVE_WAL; }
^
repl_scanner.l:98:10: note: each undeclared identifier is reported only once for each function it appears in
make[3]: *** [repl_gram.o] Error 1
make[3]: Leaving directory `/home/kaigai/repo/pgsql/src/backend/replication'
make[2]: *** [replication-recursive] Error 2
make[2]: Leaving directory `/home/kaigai/repo/pgsql/src/backend'
make[1]: *** [all-backend-recurse] Error 2
make[1]: Leaving directory `/home/kaigai/repo/pgsql/src'
make: *** [all-src-recurse] Error 2
----------------

--
NEC Business Creation Division / PG-Strom Project
KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>