build: link curl to openssl libraries when openssl support is enabled

This fixes a build failure where openssl and libmetalink are used
together and the system linker does not do implicit linking (e.g.
Fedora 13 and later releases). The MD5 functions required for
metalink support must be pulled in from the openssl crypto library.

This is similar to commit c6e7cbb9,
which fixes the same sort of problem for NSS builds.