simplify GNU Emacs builds by replacing malloc

[Paul Eggert <eggert@cs.ucla.edu>]

In Bug#27416 I propose replacing Emacs's hybrid malloc implementation on GNUish 
platforms with a hybrid implementation that redefines malloc instead of 
re-#defining it. That is, instead of defining a function hybrid_malloc that can 
call either gmalloc or the system malloc (and using the macro "#define malloc 
hybrid_malloc" in Emacs code), Emacs defines a function malloc that can call 
either gmalloc or the system malloc (via the latter’s alternate name 
‘__libc_malloc’), without using a macro.

Under this proposal, the Emacs build process no longer needs to compile library 
files twice. For example, it can simply build lib/gettime.o from lib/gettime.c, 
rather than having to build both lib/gettime.o and lib/e-gettime.o. This is 
because there is only one malloc symbol used by Emacs code, not two.

The hybrid malloc approach would not change on non-GNUish platforms.

Although this change seems like an obvious win in that it should make Emacs a 
bit easier to build and debug, I’m mentioning it on emacs-devel first in case 
anyone sees a problem with it.

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=27416