Fix build with Clang 15

Reference:	https://cgit.freebsd.org/ports/commit/astro/cfitsio/Makefile?id=7eda11c87c30c735058e9393d54b631a74cf566a

--- configure.orig	2022-10-31 18:40:23 UTC
+++ configure
@@ -5606,8 +5606,6 @@ fi
 if test "x$BUILD_REENTRANT" = xyes; then
   printf "%s\n" "#define _REENTRANT 1" >>confdefs.h
 
-  printf "%s\n" "#define _XOPEN_SOURCE 700" >>confdefs.h
-
   # Additional definition needed to get 'union semun' when using
   # _XOPEN_SOURCE on a Mac:
   if test "x$EXT" = xdarwin; then