fail to install myql gem for ruby

You tried to install mysql gem on linux:

$ gem install mysql

Building native extensions.  This could take a while...
ERROR:  Error installing mysql:
	ERROR: Failed to build gem native extension.

/home/t/ruby-1.9.2-p136-1/bin/ruby extconf.rb
checking for mysql_ssl_set()... *** 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=/home/t/ruby-1.9.2-p136-1/bin/ruby
	--with-mysql-config
	--without-mysql-config
/home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:368:in `try_do': The complier failed to generate an executable file. (RuntimeError)
You have to install development tools first.
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:435:in `try_link0'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:440:in `try_link'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:552:in `try_func'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:797:in `block in have_func'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:693:in `block in checking_for'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:280:in `block (2 levels) in postpone'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:254:in `open'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:280:in `block in postpone'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:254:in `open'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:276:in `postpone'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:692:in `checking_for'
	from /home/t/ruby-1.9.2-p136-1/lib/ruby/1.9.1/mkmf.rb:796:in `have_func'
	from extconf.rb:50:in `<main>'


Gem files will remain installed in /home/t/ruby-1.9.2-p136-1/lib/ruby/gems/1.9.1/gems/mysql-2.8.1 for inspection.
Results logged to /home/t/ruby-1.9.2-p136-1/lib/ruby/gems/1.9.1/gems/mysql-2.8.1/ext/mysql_api/gem_make.out
 

You need to check mkmf.log which is in the same folder as gem_make.out is in.  In my case, mkmf.log is in /home/t/ruby-1.9.2-p136-1/lib/ruby/gems/1.9.1/gems/mysql-2.8.1/ext/mysql_api/ 

Read mkmf.log, if you see something similar to below: 

/usr/bin/ld: skipping incompatible /usr/lib/libz.so when searching for -lz
/usr/bin/ld: skipping incompatible /usr/lib/libz.a when searching for -lz
/usr/bin/ld: cannot find -lz
collect2: ld returned 1 exit status

It means you have an incompatible zlib-devel binary.  In case, the 32-bit zlib-devel is installed, but not the 64 bits.  Run following command to fix it:

yum erase zlib-devel
yum install zlib-devel