Build-depend on libssl-dev and extract the libssl package name at build-time...

Build-depend on libssl-dev and extract the libssl package name at build-time instead of hardcoding it.

This fixes the currently broken dependency of cl-plus-ssl to libssl1.0.2 on
wheezy on apt.postgresql.org, and will avoid problems with future SONAME
changes of libssl in Debian sid.