/usr/share/boost-build/util/option.jam is in libboost1.49-dev 1.49.0-3.2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 | #  Copyright (c) 2005 Vladimir Prus.
#
#  Use, modification and distribution is subject to the Boost Software
#  License Version 1.0. (See accompanying file LICENSE_1_0.txt or
#  http://www.boost.org/LICENSE_1_0.txt)
import modules ;
# Set a value for a named option, to be used when not overridden on the command
# line.
rule set ( name : value ? )
{
    .option.$(name) = $(value) ;
}
rule get ( name : default-value ? : implied-value ? )
{
    local m = [ MATCH --$(name)=(.*) : [ modules.peek : ARGV ] ] ;
    if $(m)
    {
        return $(m[1]) ;
    }
    else  
    {        
        m = [ MATCH (--$(name)) : [ modules.peek : ARGV ] ] ;
        if $(m) && $(implied-value)
        {
            return $(implied-value) ;
        }
        else if $(.option.$(name))
        {
            return $(.option.$(name)) ;
        }    
        else
        {
            return $(default-value) ;
        }    
    }              
}
# Check command-line args as soon as possible. For each option try to load
# module named after option. Is that succeeds, invoke 'process' rule in the
# module. The rule may return "true" to indicate that the regular build process
# should not be attempted.
#
# Options take the general form of: --<name>[=<value>] [<value>]
#
rule process ( )
{
    local ARGV = [ modules.peek : ARGV ] ;
    local BOOST_BUILD_PATH = [ modules.peek : BOOST_BUILD_PATH ] ;
    local dont-build ;
    local args = $(ARGV) ;
    while $(args)
    {
        local arg = [ MATCH ^--(.*) : $(args[1]) ] ;
        while $(args[2-]) && ! $(arg)
        {
            args = $(args[2-]) ;
            arg = [ MATCH ^--(.*) : $(args[1]) ] ;
        }
        args = $(args[2-]) ;
        if $(arg)
        {
            local split = [ MATCH ^(([^-=]+)[^=]*)(=?)(.*)$ : $(arg) ] ;
            local full-name = $(split[1]) ;
            local prefix = $(split[2]) ;
            local values ;
            if $(split[3])
            {
                values = $(split[4]) ;
            }
            if $(args) && ! [ MATCH ^(--).* : $(args[1]) ]
            {
                values += $(args[1]) ;
                args = $(args[2-]) ;
            }
            # Jook in options subdirectories of BOOST_BUILD_PATH for modules
            # matching the full option name and then its prefix.
            local plugin-dir = options ;
            local option-files = [ GLOB $(plugin-dir:D=$(BOOST_BUILD_PATH)) :
                $(full-name).jam $(prefix).jam ] ;
            if $(option-files)
            {
                # Load the file into a module named for the option.
                local f = $(option-files[1]) ;
                local module-name = --$(f:D=:S=) ;
                modules.load $(module-name) : $(f:D=) : $(f:D) ;
                # If there is a process rule, call it with the full option name
                # and its value (if any). If there was no "=" in the option, the
                # value will be empty.
                if process in [ RULENAMES $(module-name) ]
                {
                    dont-build += [ modules.call-in $(module-name) : process
                        --$(full-name) : $(values) ] ;
                }
            }
        }
    }
    return $(dont-build) ;
}
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