unistd.h has to be included twice to avoid compile errors on MinGW

[Philip Nienhuis]

See this thread:
http://lists.gnu.org/archive/html/bug-gnulib/2011-01/msg00473.html
When compiling code invoking gnulib under MinGW, the statement
  #include <unistd.h>
has to present in duplo to avoid certain errors of the kind: "... is not 
a member of 'gnulib'" (where ... could be e.g., gethostname, isatty, 
unlink, etc).

This issue has not been solved until now; so here's again a request to 
look into it.

Thanks,

Philip