Como adicionar variáveis ao getconf?

Question

De acordo com a página getconf man:

NAME
      getconf — get configuration values

SYNOPSIS
       getconf [−v specification] system_var
       ...  

DESCRIPTION
       In  the first synopsis form, the getconf utility shall write to the standard output the value of the vari‐
       able specified by the system_var operand.
       ...

OPERANDS
       The following operands shall be supported:
       ...
       system_var
                 A name of a configuration variable. All of the following variables shall be supported:

                  *  The names in the Variable column of the table in the DESCRIPTION of the  sysconf()  function
                     in  the  System  Interfaces  volume of POSIX.1‐2008...

                  *  The  names  of the symbolic constants used as the name argument of the confstr() function in
                     the System Interfaces volume of POSIX.1‐2008...

                  *  The names of the symbolic constants listed under the headings ''Maximum Values'' and ''Mini‐
                     mum  Values''  in the description of the <limits.h> header in the Base Definitions volume of
                     POSIX.1‐2008...

                 The implementation may add other local values.

Como uma implementação adiciona outras variáveis locais? Existe um arquivo de configuração /etc/ , ou ele vem do ambiente de alguma forma?