# Try the following

$ gem install eventmachine -- --with-cppflags=-I/usr/local/opt/openssl/include                                                                                                             
Building native extensions with: '--with-cppflags=-I/usr/local/opt/openssl/include'
This could take a while...
Successfully installed eventmachine-1.0.8
1 gem installed

---

# If it doesn't work, try this before the above

brew install openssl