With the glibc 2.25 some functions are removed so that configure chooses
to use the malloc supplied with XEmacs. That version does not compile.
The workaround is to force configure to use system malloc. I suggest
that this is set as the default.
Probably the XEmacs supplied malloc could be removed too as dead code
but, who knows, on some old system that could still be useful!?
This could also be a problem for "core code 21.4" but I have not