[MEDIUM] Added easier support for Doug Lea's malloc (dlmalloc)

[MEDIUM] Added easier support for Doug Lea's malloc (dlmalloc)

It's now as easy as passing "DLMALLOC_SRC=<path_to_dlmalloc.c>" to
build with support for dlmalloc. The dlmalloc source is not provided
with haproxy in order to ensure that people will use either the most
recent, or the most suited version for their platform. The minimal
mmap size is specified in DLMALLOC_THRES, which defaults to 4096. It
should be increased on platforms with larger pages (eg: 8 kB on some
64 bit systems).