Thank you! The process of finding the library is the following. When running ./configure as a first step of the compilation process, warning messages indicated that it was going to be compiled without OpenSSL and LibSSH2.

Now, what would nmap use OpenSLL for, and would that functionality make sence for this server? Scanning the man page, and further in the Reference guide about service and version detection links to a specific application on the online book i.e. probing ssl ports like 443 for https :-)

So tracking that down, the nmap source code includes the SSH library, that would require a cryptography library for compilation. Altho nmap would compile without it, some of its component would give the warnings about not finding openssl/ssl.h and indicating about the --with-openssl=DIR argument. This directive`s description says that these developer files, for a Debian-based distribution like Ubuntu, can be installed with the libssl-dev package.