automake, ssl: real openssl detection, fix ssl linkage w/NSS

automake, ssl:  real openssl detection, fix ssl linkage w/NSS

This change mainly intends to prevent linking with libssl when the
crypto provider is NSS, which provides an SSL implementation (the
implementation we must use and prefer when NSS is selected).

Secondarily, actually detect openssl when it is selected, which
happens by default and co-selects with cryptopp.  To do this, we
import the ax_check_openssl.m4 from the automake archive.

Since upstream builds now prefer NSS crypto, make this the default.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>