checking whether to include SSL support... will try to find
checking for SSL_accept in -lssl... no
configure: error: OpenSSL was not found. Please supply a pathname to OpenSSL
debian:/home/bnc/eggdrop1.6.18#
Actually, assuming you're using the SSL-patch by Rootie, you'd need to use the "--with-ssl=/path/to/openssl" flag with configure.. think it's even listed when you use the --help option.
It would also seem you only installed the runtime libraries, and not the devel-packages for openssl. Without those you will never be able to compile your bot with SSL-support
"--prefix" only alters where eggdrop will be installed...