Guard non-portable forwarded includes

These headers are not available on Windows. <bsd/sys/cdefs.h> ensures
that __has_include() and __has_include_next() are defined.
Signed-off-by: Guillem Jover <guillem@hadrons.org>

include/bsd/err.h

@@ -26,20 +26,20 @@
  */
 
 #ifdef LIBBSD_OVERLAY
+#include <sys/cdefs.h>
+#if __has_include_next(<err.h>)
 #include_next <err.h>
+#endif
 #else
+#include <bsd/sys/cdefs.h>
+#if __has_include(<err.h>)
 #include <err.h>
 #endif
+#endif
 
 #ifndef LIBBSD_ERR_H
 #define LIBBSD_ERR_H
 
-#ifdef LIBBSD_OVERLAY
-#include <sys/cdefs.h>
-#else
-#include <bsd/sys/cdefs.h>
-#endif
-
 #include <stdarg.h>
 
 __BEGIN_DECLS

include/bsd/getopt.h

@@ -25,9 +25,15 @@
  */
 
 #ifdef LIBBSD_OVERLAY
+#include <sys/cdefs.h>
+#if __has_include_next(<getopt.h>)
 #include_next <getopt.h>
+#endif
 #include <unistd.h>
 #else
+#include <bsd/sys/cdefs.h>
+#if __has_include(<getopt.h>)
 #include <getopt.h>
+#endif
 #include <bsd/unistd.h>
 #endif

include/bsd/sys/endian.h

@@ -27,10 +27,16 @@
  */
 
 #ifdef LIBBSD_OVERLAY
+#include <sys/cdefs.h>
+#if __has_include_next(<endian.h>)
 #include_next <endian.h>
+#endif
 #else
+#include <bsd/sys/cdefs.h>
+#if __has_include(<endian.h>)
 #include <endian.h>
 #endif
+#endif
 
 #ifndef LIBBSD_SYS_ENDIAN_H
 #define LIBBSD_SYS_ENDIAN_H

include/bsd/sys/time.h

@@ -33,10 +33,16 @@
  */
 
 #ifdef LIBBSD_OVERLAY
+#include <sys/cdefs.h>
+#if __has_include_next(<sys/time.h>)
 #include_next <sys/time.h>
+#endif
 #else
+#include <bsd/sys/cdefs.h>
+#if __has_include(<sys/time.h>)
 #include <sys/time.h>
 #endif
+#endif
 
 #ifndef LIBBSD_SYS_TIME_H
 #define LIBBSD_SYS_TIME_H

include/bsd/unistd.h

@@ -26,19 +26,20 @@
  */
 
 #ifdef LIBBSD_OVERLAY
+#include <sys/cdefs.h>
+#if __has_include_next(<unistd.h>)
 #include_next <unistd.h>
+#endif
 #else
+#include <bsd/sys/cdefs.h>
+#if __has_include(<unistd.h>)
 #include <unistd.h>
 #endif
+#endif
 
 #ifndef LIBBSD_UNISTD_H
 #define LIBBSD_UNISTD_H
 
-#ifdef LIBBSD_OVERLAY
-#include <sys/cdefs.h>
-#else
-#include <bsd/sys/cdefs.h>
-#endif
 #include <sys/stat.h>
 
 #if !defined(S_ISTXT) && defined(S_ISVTX)