Fix build with Clang 15

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

--- configure.orig	2026-04-27 19:00:54 UTC
+++ configure
@@ -20564,8 +20564,6 @@ if test "x$BUILD_REENTRANT" = xyes; then
 if test "x$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