title: Cannot compile OCI error while installing ruby-oci8 on linux
body: <p>I got <code>cannot compile OCI</code> error while installing ruby-oci8 on linux
  the command i used is <code>gem install ruby-oci8 -v '2.2.7'</code>
  i'm using oracle full client</p>
  
  <pre><code>[root@advlints01 redf]# gem install 'ruby-oci8' -v '2.2.7'
  Building native extensions. This could take a while...
  ERROR:  Error installing ruby-oci8:
          ERROR: Failed to build gem native extension.
  
      current directory: /usr/local/rvm/gems/ruby-2.5.1@real_estate_fund/gems/ruby-oci8-2.2.7/ext/oci8
  /usr/local/rvm/rubies/ruby-2.5.1/bin/ruby -I /usr/local/rvm/rubies/ruby-2.5.1/lib/ruby/site_ruby/2.5.0 -r ./siteconf20190916-29697-1loq660.rb extconf.rb
  checking for load library path...
    LD_LIBRARY_PATH...
      checking /oracle/app/oracle/product/12.2.0/client_1/lib...   skip: /oracle/app/oracle/product/12.2.0/client_1/lib/libclntsh.so.12.1 is for i386 cpu.
    checking ld.so.conf... no
  checking for cc... ok
  checking for gcc... yes
  checking for LP64... yes
  checking for sys/types.h... yes
  checking for ruby header... ok
  Get the version of Oracle from SQL*Plus... 1220
  try  -I/oracle/app/oracle/product/12.2.0/client_1/rdbms/public
  checking for oci.h... yes
  checking for OCIEnvCreate() in oci.h... no
  *** 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/rvm/rubies/ruby-2.5.1/bin/$(RUBY_BASE_NAME)
          --with-instant-client
          --without-instant-client
  /usr/local/rvm/gems/ruby-2.5.1@real_estate_fund/gems/ruby-oci8-2.2.7/ext/oci8/oraconf.rb:755:in `initialize': RuntimeError (RuntimeError)
          from /usr/local/rvm/gems/ruby-2.5.1@real_estate_fund/gems/ruby-oci8-2.2.7/ext/oci8/oraconf.rb:269:in `new'
          from /usr/local/rvm/gems/ruby-2.5.1@real_estate_fund/gems/ruby-oci8-2.2.7/ext/oci8/oraconf.rb:269:in `get'
          from extconf.rb:22:in `&lt;main&gt;'
  ---------------------------------------------------
  Error Message:
    cannot compile OCI
  Backtrace:
    /usr/local/rvm/gems/ruby-2.5.1@real_estate_fund/gems/ruby-oci8-2.2.7/ext/oci8/oraconf.rb:755:in `initialize'
    /usr/local/rvm/gems/ruby-2.5.1@real_estate_fund/gems/ruby-oci8-2.2.7/ext/oci8/oraconf.rb:269:in `new'
    /usr/local/rvm/gems/ruby-2.5.1@real_estate_fund/gems/ruby-oci8-2.2.7/ext/oci8/oraconf.rb:269:in `get'
    extconf.rb:22:in `&lt;main&gt;'
  ---------------------------------------------------
  See:
   * http://www.rubydoc.info/github/kubo/ruby-oci8/file/docs/install-full-client.md for Oracle full client
   * http://www.rubydoc.info/github/kubo/ruby-oci8/file/docs/install-instant-client.md for Oracle instant client
   * http://www.rubydoc.info/github/kubo/ruby-oci8/file/docs/install-on-osx.md for OS X
   * http://www.rubydoc.info/github/kubo/ruby-oci8/file/docs/report-installation-issue.md to report an issue.
  
  
  To see why this extension failed to compile, please check the mkmf.log which can be found here:
  
    /usr/local/rvm/gems/ruby-2.5.1@real_estate_fund/extensions/x86_64-linux/2.5.0/ruby-oci8-2.2.7/mkmf.log
  
  extconf failed, exit code 1
  
  Gem files will remain installed in /usr/local/rvm/gems/ruby-2.5.1@real_estate_fund/gems/ruby-oci8-2.2.7 for inspection.
  Results logged to /usr/local/rvm/gems/ruby-2.5.1@real_estate_fund/extensions/x86_64-linux/2.5.0/ruby-oci8-2.2.7/gem_make.out
  [root@advlints01 redf]#
  
  
  
  
  
  </code></pre>