El Capitanでmiddlemanの環境構築しようとしたらgemのinstallがうまくいかなかったのでメモ。
bundle installしたら下のようなエラーが出た。

Using compass 1.0.3
Installing eventmachine 1.0.8 with native extensions

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    /Users/USERNAME/.rbenv/versions/2.2.1/bin/ruby -r ./siteconf20151227-26756-loa7l0.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 pipe2() in unistd.h... no
checking for accept4() in sys/socket.h... no
checking for SOCK_CLOEXEC in sys/socket.h... no
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
CFLAGS= -O3 -Wno-error=shorten-64-to-32  -pipe  -Wall -Wextra -Wno-deprecated-declarations -Wno-ignored-qualifiers -Wno-unused-result
CPPFLAGS=-I/Users/yulily100/.rbenv/versions/2.2.1/include  -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT $(DEFS) $(cppflags) -Wall -Wextra -Wno-deprecated-declarations -Wno-ignored-qualifiers -Wno-unused-result
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 files will remain installed in /Users/yulily100/.rbenv/versions/2.2.1/lib/ruby/gems/2.2.0/gems/eventmachine-1.0.8 for inspection.
Results logged to /Users/yulily100/.rbenv/versions/2.2.1/lib/ruby/gems/2.2.0/extensions/x86_64-darwin-14/2.2.0-static/eventmachine-1.0.8/gem_make.out
An error occurred while installing eventmachine (1.0.8), and Bundler cannot
continue.
Make sure that `gem install eventmachine -v '1.0.8'` succeeds before bundling.

opensslがおかしい？のでhomebrewでopensslを入れる

$ brew update
$ brew install openssl

これだけではダメで、homebrewでインストールしたOpenSSLを使うようにリンクさせる必要があります。

$ brew link openssl --force

bundle installできるようになりました。

以下を参考にしました。
https://github.com/eventmachine/eventmachine/issues/643