Re: setting include_prefix as argument to make

is there any reason not to use it?

I'm not sure if there was some specific reason not to use it initally, or if it even _was_ used initially, and then removed for some specific reason. However, one potentially complicating factor for using it is the trailing "pike" in the current default. If you specify an --includedir explicitly, would you still want "/pike" to be appended? (If yes, then it seems rather straightforward to use it actually.)

if includedir has the wrong value then we could just ignore it and set include_prefix = $(prefix)/include/pike which should be overridable just like lib_prefix

True; since the current $(include_prefix) isn't used, there is no particular need to keep the old assignment.