build: use `AS_IF` consistently in configure.ac

build: use `AS_IF` consistently in configure.ac

configure.ac contains a mix of `AS_IF` and `if` conditionals.  Prefer
the portable M4sh `AS_IF` macro.  In some cases, where there are both
`AS_IF` and `if` conditionals evaluating the same predicates, the latter
are merged into the former.

Replace three instance of `test -n "$var"` with the usual, more portable,
autoconf idiom: `test "x$var" != "x"`.

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