Removal of unexec support from glibc malloc

[Paul Eggert]

Paul Eggert wrote:
> I'll test this more, and if it doesn't have problems then we can declare the
> issue resolved, from glibc's point of view anyway.

I simulated building current Emacs on future glibc by running './configure 
emacs_cv_var_doug_lea_malloc=no', which tells 'configure' to assume that the C 
library does not have the GNU malloc extensions. Although the code still seemed 
to work, there were some compile-time errors that suggested trouble; for 
example, glibc's API for the malloc hooks disagree with Emacs's replacement. To 
fix this I would like to install the set of patches discussed for Bug#22086, here:

http://bugs.gnu.org/22086#104