Re-enable GC mark trace buffer by default

Re-enable GC mark trace buffer by default

Enable GC_REMEMBER_LAST_MARKED by default (it was disabled in Emacs 29)
to make it easier to debug difficult-to-reproduce GC problems
encountered by users.  This increases GC costs by about 5 %, which can
be avoided by turning the mark trace buffer back off using the new
--disable-gc-mark-trace option.

See discussion at
https://lists.gnu.org/archive/html/emacs-devel/2024-09/msg00240.html

* configure.ac (--disable-gc-mark-trace): New config option.
* etc/NEWS: Mention it.
* src/alloc.c: Enable it by default and avoid a compiler warning.