iptables: move XT_LOCK_NAME from CFLAGS to config.h.

This slightly simplifies configure.ac and results in more
correct dependencies.

Tested by running ./configure with --with-xt-lock-name and
without, and using strace to verify that the right lock is used.

$ make distclean-recursive && ./autogen.sh &&
  ./configure --disable-nftables --prefix /tmp/iptables &&
  make -j64 &&
  make install &&
  sudo strace -e open,flock /tmp/iptables/sbin/iptables -L foo
...
open("/run/xtables.lock", O_RDONLY|O_CREAT, 0600) = 3
flock(3, LOCK_EX|LOCK_NB)               = 0

$ make distclean-recursive && ./autogen.sh && \
  ./configure --disable-nftables --prefix /tmp/iptables \
  --with-xt-lock-name=/tmp/iptables/run/xtables.lock &&
  make -j64 &&
  make install &&
  sudo strace -e open,flock /tmp/iptables/sbin/iptables -L foo
...
open("/tmp/iptables/run/xtables.lock", O_RDONLY|O_CREAT, 0600) = 3
flock(3, LOCK_EX|LOCK_NB)               = 0

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
(cherry picked from commit 836846f0d747e1be8e37d2d43b215a68b30ea1a9)

Bug: 36108349
Test: see top of change stack.
Change-Id: I390ab17eadde6d22fa1ad9ce3a7bf4c6b2fb1b8a
2 files changed