我刚刚将我的mac升级到OS X 10.9.8。
当我谷歌这个问题时，我得到了一些解决方案。 （例如：安装Xcode，安装mysql..ect）
但是仍然无法解决这个问题。
尝试安装gem： 错误：安装mysql2时出错： 错误：无法构建gem原生扩展。
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
checking for ruby/thread.h... yes
checking for rb_thread_call_without_gvl() in ruby/thread.h... yes
checking for rb_thread_blocking_region()... yes
checking for rb_wait_for_single_fd()... yes
checking for rb_hash_dup()... yes
checking for rb_intern3()... yes
-----
Using mysql_config at /usr/local/bin/mysql_config
-----
checking for mysql.h... yes
checking for errmsg.h... yes
checking for mysqld_error.h... yes
-----
Setting rpath to /usr/local/Cellar/mysql/5.6.17_1/lib
-----
creating Makefile
make "DESTDIR=" clean
make "DESTDIR="
compiling client.c
compiling infile.c
compiling mysql2_ext.c
compiling result.c
linking shared-object mysql2/mysql2.bundle
clang: error: unknown argument: '-multiply_definedsuppress' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future
make: *** [mysql2.bundle] Error 1
make failed, exit code 2
Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/mysql2-0.3.15 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/extensions/universal-darwin-13/2.0.0/mysql2-0.3.15/gem_make.out
解决方案是sudo ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future gem install mysql2。
如果有人遇到同样的问题，您可以尝试ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future gem install gem_name。