mysql-server-5.1.19 path variable error created on upgrade compilation

[David Southwell]

# uname -a
 6.1-RELEASE FreeBSD 6.1-RELEASE #0: Sun May  7 04:15:57 UTC 2006     
root at bloom.cse.buffalo.edu:/usr/obj/usr/src/sys/SMP  amd64
#
Just upgraded mysql to include:

# pkg_info |grep mysql
mysql-client-5.1.19 Multithreaded SQL database (client)
mysql-server-5.1.19 Multithreaded SQL database (server)
p5-DBD-mysql51-4.005 MySQL 5.1 driver for the Perl5 Database Interface (DBI)
php5-mysql-5.2.3    The mysql shared extension for php
#

Seems we have an oddity here after compiling:
Extract from show variables:
show variables;
+---------------------------------+---------------------------------------+
| Variable_name                   | Value                                 |
+---------------------------------+---------------------------------------+
| basedir                         | /usr/local/  
| character_sets_dir              | /usr/local/share/mysql/charsets/      |
                           
| datadir                         | /usr2/datadb/                         |

| general_log_file                | /usr2/datadb/dns1.log                 |

| language                        | /usr/local/share/mysql/english/       |

| pid_file                        | /usr2/datadb//dns1.vizion2000.net.pid |

                                                         ^^----Why '//'

| plugin_dir                      | /usr/local/lib/mysql                  |

| slow_query_log_file             | /usr2/datadb/dns1-slow.log            |
| socket                          | /tmp/mysql.sock                       |

So I thought I would:
(a) RESET the variable

mysql> set pid_file = /usr2/datadb/dns1.vizion2000.net.pid ;
ERROR 1193 (HY000): Unknown system variable 'pid_file'
mysql> set GLOBAL pid_file = /usr2/datadb/dns1.vizion2000.net.pid ;
ERROR 1193 (HY000): Unknown system variable 'pid_file'
mysql> set GLOBAL pid_file = '/usr2/datadb/dns1.vizion2000.net.pid' ;
ERROR 1193 (HY000): Unknown system variable 'pid_file'
mysql> set @@GLOBAL pid_file = '/usr2/datadb/dns1.vizion2000.net.pid' ;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual 
that corresponds to your MySQL server version for the right syntax to use 
near 'pid_file = '/usr2/datadb/dns1.vizion2000.net.pid'' at line 1
mysql> set @@GLOBAL.pid_file = '/usr2/datadb/dns1.vizion2000.net.pid' ;
ERROR 1193 (HY000): Unknown system variable 'pid_file'
mysql>  

But I cannot seem to get the Syntax right - can someone please point me in the 
right direction.

(b) Ask why this might be happening.

It might be worth recording that /usr2/ is a seperate physical device and 
emphasize that the base-dir is /usr/local/.

Thanks in advance

David