Added compilation comment to 'show variables'

This makes it possible to check origin of binary from the output of 'show variables'

include/mysql_version.h.in

@@ -19,6 +19,7 @@
 #define MYSQL_PORT			@MYSQL_TCP_PORT@
 #define MYSQL_UNIX_ADDR			"@MYSQL_UNIX_ADDR@"
 #define MYSQL_CONFIG_NAME		"my"
+#define MYSQL_COMPILATION_COMMENT	"@COMPILATION_COMMENT@"
 
 /* mysqld compile time options */
 #ifndef MYSQL_CHARSET

sql/mysqld.cc

@@ -4278,8 +4278,8 @@ struct show_var_st status_vars[]= {
 
 static void print_version(void)
 {
-  printf("%s  Ver %s for %s on %s\n",my_progname,
-	 server_version,SYSTEM_TYPE,MACHINE_TYPE);
+  printf("%s  Ver %s for %s on %s (%s)\n",my_progname,
+	 server_version,SYSTEM_TYPE,MACHINE_TYPE, MYSQL_COMPILATION_COMMENT);
 }
 
 static void use_help(void)

sql/set_var.cc

@@ -593,6 +593,7 @@ struct show_var_st init_vars[]= {
    SHOW_SYS},
   {sys_trans_prealloc_size.name, (char*) &sys_trans_prealloc_size,  SHOW_SYS},
   {"version",                 server_version,                       SHOW_CHAR},
+  {"version_comment",         (char*) MYSQL_COMPILATION_COMMENT,    SHOW_CHAR},
   {sys_net_wait_timeout.name, (char*) &sys_net_wait_timeout,	    SHOW_SYS},
   {NullS, NullS, SHOW_LONG}
 };