[ruby] gem install 시 openssl 설치시 file not found 해결하기

맥에서 eventmachine 설치시 open ssl을 잘 읽지 못해서 에러가 발생할 수 있다. 

$ gem install eventmachine -v '1.0.8'

compiling binder.cpp

In file included from binder.cpp:20:

./project.h:116:10: fatal error: 'openssl/ssl.h' file not found

#include <openssl/ssl.h>

         ^

1 error generated.

make: *** [binder.o] Error 1

make failed, exit code 2

에러 발생하면, 

$ brew install openssl

$ xcode-select --install

그래도 안되면.. 강제로 link 한다. 

$ brew link openssl --force

그리고, 다시 설치하면 잘 동작한다. 

$ gem install eventmachine -v '1.0.8

'Building native extensions.  This could take a while...

Successfully installed eventmachine-1.0.8

Parsing documentation for eventmachine-1.0.8

Installing ri documentation for eventmachine-1.0.8

Done installing documentation for eventmachine after 5 seconds

1 gem installed