[mod_webdav] copy_file_range() new in FreeBSD 13

(thx devnexen)

adjust feature defines for header visibility of copy_file_range()
(introduced in FreeBSD 13)

x-ref:
  https://redmine.lighttpd.net/issues/3128
  https://github.com/freebsd/freebsd-src/blob/main/sys/sys/cdefs.h#L634

src/mod_webdav.c

@@ -169,6 +169,13 @@
 #ifndef _GNU_SOURCE
 #define _GNU_SOURCE
 #endif
+#ifdef HAVE_COPY_FILE_RANGE /* FreeBSD 13+ */
+#ifdef __FreeBSD__ /* FreeBSD strict compliance hides libc extensions */
+/*#define _XOPEN_SOURCE 700*//*(defined above)*/
+#define _ISOC11_SOURCE
+#define __BSD_VISIBLE 1
+#endif
+#endif
 
 #include "first.h"      /* first */
 #include "sys-mmap.h"