Fix Autoconf deprecations

Changes suggested by autoupdate configure.ac.in.

@skunnyk Could you please have a look at this MR? (I've seen you've done several MRs of this type for various components lately.)

Autoconf started showing me warnings these days, recommending me to run autoupdate, which I did. It eliminated a few of these warnings, but most of them stayed because they come from configure.ac, which is autogenerated.

I don't know if something should be done now for this, or if we should wait...