As described in the previous commit, GCC 15 uses C23 by default and that
changes the meaning of such argument-less function declarations. So
whenever we assign such a function to a pointer that expects a function
with arguments it causes an incompatible pointer type warning. We
could define dedicated functions/callbacks whenever necessary, but this
seems like the simpler approach for now (especially since most uses of
these functions have already been cast).
INCLUDES are now deprecated and throw warnings when using automake 1.13.
We now also differentiate AM_CPPFLAGS and AM_CFLAGS, where includes and
defines are passed to AM_CPPFLAGS only.
Tobias Brunner