Clean up the fix for unexec build on GNU/Linux

Clean up the fix for unexec build on GNU/Linux

* src/conf_post.h [HYBRID_MALLOC || DARWIN_OS && HAVE_UNEXEC]:
Include <stdlib.h> here, before redirecting 'malloc' and friends
to their hybrid_* and unexec_* equivalents.  #undef malloc and
friends before redefining.  Provide prototypes for the
replacements.  Suggested by Paul Eggert <eggert@cs.ucla.edu>.
* src/gmalloc.c [HYBRID_MALLOC]: Remove declarations of 'malloc'
and friends, as they are now redundant: we include <stdlib.h> in
conf_post.h before redefining 'malloc' etc., and that provides
prototypes from system headers.

* configure.ac (HYBRID_MALLOC): Remove kludge to avoid replacement
of 'free' by Gnulib.  (Bug#36649)