Go back to preferring -isystem to -I

Go back to preferring -isystem to -I

* configure.ac: Go back to preferring -isystem to -I,
as headers like <gobject/gparam.h> still need it.  This
reverts almost all of 2024-07-16T02:25:44!eggert@cs.ucla.edu,
except that the ‘nw="$nw -Wsystem-headers"’ line continues to
be removed as it is no longer needed due to recent Gnulib changes.
Problem reported by Eli Zaretskii in:
https://lists.gnu.org/r/emacs-devel/2024-07/msg00756.html

(cherry picked from commit 3a790abd869ddadc343710deb0c4368227ba6611)