gem install eventmachine時に「'openssl/ssl.h' file not found」とエラー

環境

• Mac El Capitan

Gemfileに

症状

gem install eventmachine ができない

gem "eventmachine"

でbundle instrallすると

Using ref 1.0.5
Using sass 3.2.14
Using simple_oauth 0.3.0
Using elasticsearch-api 1.0.1
Using thread_safe 0.2.0
Using airbrake 4.0.0
Using rack-test 0.6.2
Using treetop 1.4.15
Using rspec-core 3.1.7
Using rspec-mocks 3.1.3
Using rspec-expectations 3.1.2
Using better_errors 2.0.0
Using net-scp 1.2.1
Using whenever 0.9.4
Using coffee-script 2.2.0
Using unf 0.1.4
Using faraday 0.9.0
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    /Users/thr3a/.rbenv/versions/2.2.4/bin/ruby -r ./siteconf20170322-53700-z08h1b.rb extconf.rb 
checking for rb_trap_immediate in ruby.h,rubysig.h... no
checking for rb_thread_blocking_region()... no
checking for ruby/thread.h... yes
checking for rb_thread_call_without_gvl() in ruby/thread.h... yes
checking for inotify_init() in sys/inotify.h... no
checking for __NR_inotify_init in sys/syscall.h... no
checking for writev() in sys/uio.h... yes
checking for rb_thread_fd_select()... yes
checking for rb_fdset_t in ruby/intern.h... yes
checking for rb_wait_for_single_fd()... yes
checking for rb_enable_interrupt()... no
checking for rb_time_new()... yes
checking for sys/event.h... yes
checking for sys/queue.h... yes
checking for clock_gettime()... no
checking for gethrtime()... no
creating Makefile

make "DESTDIR=" clean

make "DESTDIR="
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

と怒られる

対処法

 gem install eventmachine -- --with-cppflags=-I/usr/local/opt/openssl/include  

Gemfile.lockのバージョンに合わせてインストールするには

gem install eventmachine -v "1.0.7" -- --with-cppflags=-I/usr/local/opt/openssl/include

でいける