Work around make 3.80 bug with long expansions of $(eval).

3.80 breaks if the expansion of $(eval) is long enough to require expansion
of its internal variable_buffer.  For the purposes of $(recurse) that means
it'll work so long as no single evaluation of _create_recursive_target
produces more than 195 bytes.  We can manage that by looping over
subdirectories outside the call instead of complicating the generated rule.
This coding is simpler and more readable anyway.

Or at least, this works for me.  We'll see if the buildfarm likes it.