Added call to mysql_library_init during initialization of the gem

  This call must be performed before trying to call mysql_init from multiple threads
  Reference: http://dev.mysql.com/doc/refman/5.1/en/mysql-init.html
  Minimal reproduction of the problem if mysql_library_init is not called

    require 'mysql2'

    def connect
      Mysql2::Client.new()
    end

    threads = [0,1].map {
      Thread.new { connect }
    }
    threads.map(&:join)
    puts "OK!"

Author: michaelkruglos

ext/mysql2/client.c

@@ -1235,6 +1235,13 @@ void init_mysql2_client() {
     }
   }
 
+  /* Initializing mysql library, so different threads could call Client.new */
+  /* without race condition in the library */
+  if (mysql_library_init(0, NULL, NULL) != 0) {
+    rb_raise(rb_eRuntimeError, "Could not initialize MySQL client library");
+    return;
+  }
+
 #if 0
   mMysql2      = rb_define_module("Mysql2"); Teach RDoc about Mysql2 constant.
 #endif