gem eventmachine fatal error: 'openssl/ssl.h' file not found

Just installed El Capitan and can't install gem eventmachine 1.0.7. openssl is at 1.0.2a-1. Tried to use --with-ssl-dir but it seems ignored.

Reported it to their github repo as well.

Any suggestions are really appreciated. Thanks.

$ ls /usr/local/Cellar/openssl/1.0.2a-1/include/openssl/ssl.h
/usr/local/Cellar/openssl/1.0.2a-1/include/openssl/ssl.h

$ gem install eventmachine -v '1.0.7' -- --with-ssl-dir=/usr/local/Cellar/openssl/1.0.2a-1/include
/Users/pain/.rbenv/versions/2.1.2/bin/ruby -r ./siteconf20150612-56154-1hsjz2n.rb extconf.rb --with-ssl-dir=/usr/local/Cellar/openssl/1.0.2a-1/include
checking for rb_trap_immediate in ruby.h,rubysig.h... no
checking for rb_thread_blocking_region()... yes
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... 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

$ 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

You can also set up bundler like this but I think that is superficial

bundle config build.eventmachine --with-cppflags=-I/usr/local/opt/openssl/include

When using bundler and homebrew:

$ bundle config build.eventmachine --with-cppflags=-I$(brew --prefix openssl)/include
$ bundle install