Compile-based feature detection (e.g. xtables, ipset support) was broken
due to silent compilation errors in the configure script, caused by a
Makefile variable KERNEL_INCLUDE referring to kernel build headers. Use
userspace headers by setting the same "user_headers" kernel include path
as used for the iptables build.

Remove redundant or unused Build/Configure definitions from package
Makefile, including KERNEL_INCLUDE, LIBC_INCLUDE and DBM includes.

Don't pass LDFLAGS within MAKE_FLAGS as this interferes with LDFLAGS in
tc/Makefile and masks a link parameter ("-Wl,-export-dynamic"). Instead,
use standard TARGET_LDFLAGS.

Replace EXTRA_CCOPTS in MAKE_FLAGS with cleaner TARGET_CPPFLAGS, and also
drop now unneeded patch 150-extra-ccopts.patch.

Enable defining XT_LIB_DIR from Makefile, needed to set the iptables
modules directory to something other than /lib/xtables, and also add
libxtables dependency. Both are needed with working xtables detection.
Note that libxtables is also pulled in by iptables, firewall or luci, so
this change has no size impact in most cases.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>