bug#18368: HYBRID_MALLOC broke AIX build

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

The trunk's recent HYBRID_MALLOC changes broke the build on AIX.  Here 
are the first couple of compile-time diagnostics:

xlc -c  -Demacs  -I. -I. -I../lib -I../lib -g3 -O  gmalloc.c
"/usr/include/unistd.h", line 201.17: 1506-343 (S) Redeclaration of 
lseek64 differs from previous declaration on line 199 of 
"/usr/include/unistd.h".
"/usr/include/unistd.h", line 201.17: 1506-050 (I) Return type "long 
long" in redeclaration is not compatible with the previous return type 
"long".

The culprit is this part of gmalloc.c:

/* If HYBRID_MALLOC is defined in config.h, then conf_post.h #defines
    malloc and friends as macros before including stdlib.h.  In this
    file we will need the prototypes for the system malloc, so we must
    include stdlib.h before config.h.  And we have to do this
    unconditionally, since HYBRID_MALLOC hasn't been defined yet.  */
#include <stdlib.h>

#include <config.h>


This doesn't work, as <config.h> must come first for various reasons; I 
suspect the AIX breakage is just the tip of the iceberg.

One possible fix is to declare the system malloc directly, if 
HYBRID_MALLOC is defined.  Another one, which might be safer, is to 
modify config.h to capture the system malloc before it's redefined.

Since AIX does not define HYBRID_MALLOC, I have temporarily worked 
around the problem on my AIX host by using this in gmalloc.c:

#ifdef HYBRID_MALLOC
#include <stdlib.h>
#endif

#include <config.h>

#include <stdlib.h>

but I suspect this is not the right way to fix the problems on platforms 
that need HYBRID_MALLOC but have AIX-like problems with redefinitions.