build: fix autoconf warnings

build: fix autoconf warnings

autoconf complains about three obsolete macros.

`AC_CONFIG_HEADER` has been superseded by `AC_CONFIG_HEADERS`, so
replace it.

`AM_PROG_LEX` calls `AC_PROG_LEX` with no arguments, but this usage is
deprecated.  The only difference between `AM_PROG_LEX` and `AC_PROG_LEX`
is that the former defines `$LEX` as "./build-aux/missing lex" if no lex
is found to ensure a useful error is reported when make is run.  How-
ever, the configure script checks that we have a working lex and exits
with an error if none is available, so `$LEX` will never be called and
we can replace `AM_PROG_LEX` with `AC_PROG_LEX`.

`AM_PROG_LIBTOOL` has been superseded by `LT_INIT`, which is already in
configure.ac, so remove it.

We can also replace `AC_DISABLE_STATIC` with an argument to `LT_INIT`.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>