Remove malloc.h (#13436)

This removes the deprecated malloc.h header Autoconf check on *nix
systems and its HAVE_MALLOC_H symbol. It can be replaced mostly with the
stdlib.h. The libgd usptream also doesn't include it anymore.

On Windows, it is still used for some memory allocation functions, but
can be replaced with stdlib.h in the future.

configure.ac

@@ -398,7 +398,6 @@ grp.h \
 ieeefp.h \
 langinfo.h \
 linux/sock_diag.h \
-malloc.h \
 os/signpost.h \
 poll.h \
 pty.h \

ext/gd/libgd/gdcache.h

@@ -41,9 +41,6 @@
 /*********************************************************/
 
 #include <stdlib.h>
-#if (!defined(__OpenBSD__)) && defined(HAVE_MALLOC_H)
- #include <malloc.h>
-#endif
 #ifndef NULL
 #define NULL (void *)0
 #endif