Avoid a compiler warning about integer overflow when compiling this
for 32 bit targets.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13477 a5019735-40e9-0310-863c-91ae7b9d1cf9
Define _XOPEN_SOURCE before including unistd.h.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13478 a5019735-40e9-0310-863c-91ae7b9d1cf9