Added Doug Lea's malloc, use --enable-dlmalloc to enable it. It's enabled

by default on Windows since the OS malloc sucks.

Rev: src/acconfig.h:1.153
Rev: src/configure.in:1.988
Rev: src/dmalloc.h:1.60
Rev: src/malloc.c:1.1
Rev: src/pike_memory.c:1.180