Build: modernize CURL library detection
Use package config instead of checking headers.
Package config is faster because it does not invoke compiler
and guarantees proper linking flags because these are provided
by package maintainer instead of hardcoded into build system.
https://fedorahosted.org/freeipa/ticket/6418
Reviewed-By: Lukas Slebodnik <lslebodn@redhat.com>
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>