title: An error occurred while installing mysql2 (0.3.21), and Bundler cannot continue
body: <p>Any reason why this error popped up when I tried bundling an application:
  I have tried installing gem install mysql2 -v '0.3.21' as they recommend but it cant install properly. Also I am running this on macOS High Sierra. Sorry for my bad wording for this question because its my first time working with ruby.</p>
  
  <pre><code> To see why this extension failed to compile, please check the mkmf.log which can be found here:
  
    /Users/yamanshrestha/Desktop/Dorsata/vendor/bundle/ruby/2.3.0/extensions/universal-darwin-17/2.3.0/mysql2-0.3.21/mkmf.log
  
  current directory: /Users/yamanshrestha/Desktop/Dorsata/vendor/bundle/ruby/2.3.0/gems/mysql2-0.3.21/ext/mysql2
  make "DESTDIR=" clean
  
  current directory: /Users/yamanshrestha/Desktop/Dorsata/vendor/bundle/ruby/2.3.0/gems/mysql2-0.3.21/ext/mysql2
  make "DESTDIR="
  compiling infile.c
  compiling client.c
  client.c:439:3: error: use of undeclared identifier 'my_bool'
    my_bool res = mysql_read_query_result(client);
    ^
  client.c:441:19: error: use of undeclared identifier 'res'
    return (void *)(res == 0 ? Qtrue : Qfalse);
                    ^
  client.c:762:3: error: use of undeclared identifier 'my_bool'
    my_bool boolval;
    ^
  client.c:793:7: error: use of undeclared identifier 'boolval'
        boolval = (value == Qfalse ? 0 : 1);
        ^
  client.c:794:17: error: use of undeclared identifier 'boolval'
        retval = &amp;boolval;
                  ^
  client.c:797:10: error: use of undeclared identifier 'MYSQL_SECURE_AUTH'; did you mean 'MYSQL_DEFAULT_AUTH'?
      case MYSQL_SECURE_AUTH:
           ^~~~~~~~~~~~~~~~~
           MYSQL_DEFAULT_AUTH
  /usr/local/Cellar/mysql/8.0.11/include/mysql/mysql.h:188:3: note: 'MYSQL_DEFAULT_AUTH' declared here
    MYSQL_DEFAULT_AUTH,
    ^
  client.c:798:7: error: use of undeclared identifier 'boolval'
        boolval = (value == Qfalse ? 0 : 1);
        ^
  client.c:799:17: error: use of undeclared identifier 'boolval'
        retval = &amp;boolval;
                  ^
  client.c:830:38: error: use of undeclared identifier 'boolval'
          wrapper-&gt;reconnect_enabled = boolval;
                                       ^
  client.c:1185:38: error: use of undeclared identifier 'MYSQL_SECURE_AUTH'; did you mean 'MYSQL_DEFAULT_AUTH'?
    return _mysql_client_options(self, MYSQL_SECURE_AUTH, value);
                                       ^~~~~~~~~~~~~~~~~
                                       MYSQL_DEFAULT_AUTH
  /usr/local/Cellar/mysql/8.0.11/include/mysql/mysql.h:188:3: note: 'MYSQL_DEFAULT_AUTH' declared here
    MYSQL_DEFAULT_AUTH,
    ^
  10 errors generated.
  make: *** [client.o] Error 1
  
  make failed, exit code 2
  
  Gem files will remain installed in /Users/yamanshrestha/Desktop/Dorsata/vendor/bundle/ruby/2.3.0/gems/mysql2-0.3.21 for inspection.
  Results logged to /Users/yamanshrestha/Desktop/Dorsata/vendor/bundle/ruby/2.3.0/extensions/universal-darwin-17/2.3.0/mysql2-0.3.21/gem_make.out
  
  An error occurred while installing mysql2 (0.3.21), and Bundler cannot continue.
  Make sure that `gem install mysql2 -v '0.3.21' --source 'http://rubygems.org/'` succeeds before bundling.
  </code></pre>