BUILD: Makefile: require SSL_LIB, SSL_INC to be explicitly set

BUILD: Makefile: require SSL_LIB, SSL_INC to be explicitly set

The SSL_INC and SSL_LIB variables were not initialized in the Makefile,
so they could be accidently inherited from the environment. We require
that any makefile variable is explicitly set on the command line so they
must be initialized.

Note that the Travis scripts used to rely only on these variables to be
exported, so it was adjusted as well.