Official C driver for MongoDB Formula was removed
Last versions were
mongo-c requires the following formulae to be installed:
pkg-config 0.29.2 Manage compile and link flags for libraries
openssl 1.0.2n SSL/TLS cryptography library

Recent formula history

Mike McQuaid mongo-c-driver: rename from mongo-c, cleanup.
ilovezfs mongo-c 1.5.0
Mike McQuaid Use hash rockets again. (#5177)
Mike McQuaid Use Ruby 1.9+ symbol hash keys in all formulae. (#4942)
ilovezfs mongo-c 1.4.0

Formula code at GitHub