configure: fix the -ldl check for openssl, add -lpthread check

The check for if -ldl is needed to build with (a statically built)
openssl was broken. This repairs the check, and adds a check for
-lpthread as well since OpenSSL 1.1.0+ does in fact require -lpthread so
only adding -ldl for a static openssl build is no longer enough.

Reported-by: Jay Satiro
Ref: #1426
Closes #1427