Include protection for <unistd.h>, <sys/types.h> and <sys/stat.h>.

diff --git a/tools/widl/proxy.c b/tools/widl/proxy.c
index a0677bd..2cf3ff1 100644
--- a/tools/widl/proxy.c
+++ b/tools/widl/proxy.c
@@ -22,7 +22,9 @@
 
 #include <stdio.h>
 #include <stdlib.h>
-#include <unistd.h>
+#ifdef HAVE_UNISTD_H
+# include <unistd.h>
+#endif
 #include <string.h>
 #include <assert.h>
 #include <ctype.h>