Configure HTTPS/libcurl by default because it's now needed to access CRAM reference registry

[mlin]

Per @tk2, EBI recently began switching all www connections, including to the CRAM Reference Registry, to HTTPS. This is good even though the references are public data, because it helps to ensure the data can't be corrupted in-flight.

However, it means that htslib now requires HTTPS support (i.e. libcurl, ./configure --enable-libcurl) in order to access the CRAM Reference Registry. Otherwise (by default!), viewing a CRAM file fails with a cryptic error "Failed to populate reference for id ..."

My suggestion therefore would be to change libcurl from opt-in to opt-out.