Workaround headers problem in Darwin 8 as well.

--- rts/PosixSource.h
+++ rts/PosixSource.h
@@ -11,6 +11,13 @@
 
 #include <ghcplatform.h>
 
+#if defined(darwin_HOST_OS)
+/* unfortunately the hack below only works for Darwin 9 and
+ * above, so as hack include sys/types early (before setting
+ * _POSIX_C_SOURCE) (this obviously obsoletes the hack below) */
+#include <sys/types.h>
+#endif
+
 #if defined(freebsd_HOST_OS)
 #define _POSIX_C_SOURCE 200112L
 #define _XOPEN_SOURCE   600