1.ERROR:  While executing gem ... (Gem::Exception)    Unable to require openssl, install OpenSSL and rebuild ruby (preferred) or use non-HTTPS sources

方法：

进入ruby源码解压包，进入ruby 源码包下的  ext/openssl ，用ruby自带的openssl编译，而不是yum 的openssl

cd /root/ruby-2.1.5/ext/openssl
ruby extconf.rb
成功...
make
报错....
make: *** No rule to make target `/thread_native.h', needed by `ossl.o'.  Stop.


解决方法：
打开Makefile
vim Makefile
加入第三行路径，也就是【top_srcdir = /root/ruby-2.1.5】
srcdir = .
topdir = /usr/local/ruby/include/ruby-2.1.0
top_srcdir = /root/ruby-2.1.5
hdrdir = $(topdir)

保存退出，make clean，
重新make,成功，

make install 成功

[root@mysql openssl]# make 
linking shared-object openssl.so
[root@mysql openssl]# make install
/usr/bin/install -c -m 0755 openssl.so /usr/local/ruby/lib/ruby/site_ruby/2.1.0/x86_64-linux
installing default openssl libraries

2.

 gem install mysql2

Fetching: mysql2-0.4.3.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing mysql2:
	ERROR: Failed to build gem native extension.

    /usr/local/ruby/bin/ruby -r ./siteconf20160321-3049-192s9jh.rb 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
checking for mysql_query() in -lmysqlclient... no
-----
mysql client is missing. You may need to 'apt-get install libmysqlclient-dev' or 'yum install mysql-devel', and try again.
-----
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--without-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/usr/local/ruby/bin/ruby
	--with-mysql-dir
	--without-mysql-dir
	--with-mysql-include
	--without-mysql-include=${mysql-dir}/include
	--with-mysql-lib
	--without-mysql-lib=${mysql-dir}/lib
	--with-mysql-config
	--without-mysql-config
	--with-mysql-dir
	--without-mysql-dir
	--with-mysql-include
	--without-mysql-include=${mysql-dir}/include
	--with-mysql-lib
	--without-mysql-lib=${mysql-dir}/lib
	--with-mysqlclientlib
	--without-mysqlclientlib

已经讲的很清楚了，安装mysql-devel

yum install mysql-devel