Static cmocka library

Early in 2021, CS50 revoked sudoer access (previously the ubuntu user had been in the sudoers group and used a blank password). This prevented the installation of libraries and packages via apt-get.

This release is a potential workaround that includes a pre-compiled static library of cmocka (version 1.1.5, released 2019-03-28). The library was compiled and committed on 2021-05-21 on the CS50 IDE which (at the time) was running Ubuntu 20.04.2 LTS.

Workaround instructions/explanations were included in comments in the makefile. Ultimately the makefile was modified so that no instructions in the lab handout needed to be modified.

This is still a fragile workaround: the static library may need to be recompiled for subsequent updates to CS50/Cloud9/etc. CS50 admins have been engaged in order to request that cmocka and other libraries be included in the default docker image. A response is pending as of 2021/05/21.