configure: Support empty target list (--target-list=)

Specifying an empty target list with --target-list= is shorter
than specifying --disable-user --disable-system.

Both variants should give the same result: no targets at all.

This modification implements that feature.

It uses a trick which works with POSIX compliant shells to test whether
target_list is undefined (=> default targets) or empty (=> no targets).

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>