[squid-users] dlmalloc.c:796: conflicting types for `sbrk'

I am attempting to compile Squid2.4Stable3 with dlmalloc on FreeBSD
4.5. Running 'configure' works fine. However, when running 'make', I get
the following error:

---
dlmalloc.c:796: conflicting types for `sbrk'
/usr/include/unistd.h:185: previous declaration of `sbrk'
---
Is this a known problem? Can anyone help me fix this? The other 
'configure' options I am using are:
    --enable-ipf-transparent
    --enable-underscores
    --enable-snmp
    --enable-removal-policies=heap
They all work fine without --enable-dlmalloc.
Thanks,
Lucas