can't compile using --enable-dlmalloc

hello everyone,

i got this error while compiling 1.2b25 on a freebsd 2.2.7-stable machine
with --enable-dlmalloc option:

        dlmalloc.c:792: conflicting types for `sbrk'
        /usr/include/unistd.h:156: previous declaration of `sbrk'

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