[PATCH] Fix compilation --with-debug-malloc && --with-error-checking=malloc; better dlmalloc overhead

[Stephen J. Turnbull]

Nix writes:

 > As an additional wrinkle, the Doug Lea malloc() has always had an
 > overhead of at least 4 bytes per allocated chunk (8 bytes per chunk for
 > machines with 8-bit pointers, more if this is a large request satisfied
 > via mmap() rather than via arena allocation). For now, 4 bytes is more
 > accurate than the previous figure of, um, 0. (What malloc() has no bytes
 > of overhead?! How on earth would it track blocks?)

ISTR comments indicating that malloc overhead was handled in some sly
way.  Am I misremembering?

I've always thought it absurd that you can't ask the system for
information about a block if you hand it a pointer.