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

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?)