cgen: add autofree comptime check

[felipensp]

Add possibility to check if autofree is enabled.

$if autofree {

}

[spytheman]

Excellent work.

[spytheman]

There seems to be one problem:

println('start')

$if autofree {
    println('autofree was passed')
}
$if !autofree {
    println('no autofree')
}

$if abc ? {
    println('passed `-d abc`')
}

$if !abc ? {
    println('no `-d abc` passed')
}

println('done')

The no autofree case is never triggered for some reason.