Include header check for unistd.h in utils.c
Include header check for unistd.h in utils.c

The header unistd.h is not available on on platforms, so we add a check
around it.

ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz