find_in_given_path(): not handling directories properly

[Paul Smith]

In gnulib commit 7b1de4a62f876:

  commit 7b1de4a62f8766787160f785217671b074e0f0f2
  Author: Bruno Haible <address@hidden>
  Date:   2020-04-10 15:57:10 +0200

      findprog, relocatable-prog: Ignore directories during PATH search.

      Reported by Frederick Eaton via Dmitry Goncharov in
      <https://lists.gnu.org/archive/html/bug-gnulib/2020-03/msg00003.html>.

The lib/findprog.c implementation of the find_in_path() function was
fixed by this commit.

However, the same issue exists in lib/findprog-in.c, in the
find_in_given_path() (which is what GNU make actually uses), and that
function wasn't updated by the above commit.

Cheers!