I did a bundle install on a Rails project and got a bunch of errors dealing with openssl when it tries to compile. Towards the end of the message, we see:

#warning use "ruby/io.h" instead of "rubyio.h"
mini_ssl.c:4:10: fatal error: 'openssl/bio.h' file not found
#include <openssl/bio.h>
1 warning and 1 error generated.
make: *** [mini_ssl.o] Error 1

make failed, exit code 2

The solution is to

$ brew install openssl
$ brew link --force openssl

Even if you have openssl installed, make sure to run the link command otherwise bundler won’t see it.