Removal of unexec support from glibc malloc

Hi,

this is a heads-up that glibc malloc will likely remove support code for
the Emacs unexec mechanism during the coming year.  The reason is that
we want to be able to change heap layout between glibc releases, for
standards conformance, performance and security improvements:

  <https://sourceware.org/bugzilla/show_bug.cgi?id=19473>

This only affects the glibc API, so Emacs is unlikely to compile from
source until you remove the undump mechanism.  We will work very hard to
keep existing Emacs binaries working, but we might miss some corner
cases initially.  Those will be treated as bugs and fixed (although this
causes a significant ongoing maintenance overhead for glibc upstream,
basically maintaining a separate malloc implementation indefinitely).

Florian