When attempting to install libevent, I got the following error:
fatal error: 'openssl/bio.h' file not found
To correct it, I already had openssl installed via brew:
➜ brew install openssl
Warning: openssl-1.0.2a-1 already installed
Since it was already installed, I needed to install the openssl headers into /usr/local/inclue/openssl.
➜ brew link openssl --force
Linking /usr/local/Cellar/openssl/1.0.2a-1... 1543 symlinks created
Now libevent succeeded on the subsequent 'make' run.