http: Always check for "Connection: close"  presence
use pkg-config to check zlib deps if available

Newer versions of these packages ship with pkg-config files, so if we can
detect it via those, do so.  If that fails, fall back to the old methods.
use pkg-config to check libuuid deps if available

Newer versions of these packages ship with pkg-config files, so if we can
detect it via those, do so.  If that fails, fall back to the old methods.

Also add a configure flag to explicitly control its usage.
use pkg-config to check pcre deps if available

Newer versions of these packages ship with pkg-config files, so if we can
detect it via those, do so.  If that fails, fall back to the old methods.

Also add a configure flag to explicitly control its usage.
use pkg-config to check openssl deps if available

Newer versions of these packages ship with pkg-config files, so if we can
detect it via those, do so.  If that fails, fall back to the old methods.
use pkg-config to check gnutls deps if available

Newer versions of these packages ship with pkg-config files, so if we can
detect it via those, do so.  If that fails, fall back to the old methods.
NEWS: add pkg-config support