[PATCH nft v2 1/6] meta: define _GNU_SOURCE to get strptime() from <time.h>

[Thomas Haller <thaller@redhat.com>]

To use `strptime()`, the documentation indicates

  #define _XOPEN_SOURCE
  #include <time.h>

However, previously this was done wrongly.

For example, when building with musl we got a warning:

      CC       meta.lo
    meta.c:40: warning: "_XOPEN_SOURCE" redefined
       40 | #define _XOPEN_SOURCE
          |
    In file included from /usr/include/errno.h:8,
                     from meta.c:13:
    /usr/include/features.h:16: note: this is the location of the previous definition
       16 | #define _XOPEN_SOURCE 700
          |

Defining "__USE_XOPEN" is wrong. This is a glibc internal define not for
the user.

Note that if we just set _XOPEN_SOURCE (or _XOPEN_SOURCE=700), we won't
get other things like "struct tm.tm_gmtoff".

Instead, we already define _GNU_SOURCE at other places. Do that here
too, it will give us strptime() and all is good.

Also, those directives should be defined as first thing (or via "-D"
command line). See [1].

This is also important, because to use "time_t" in a header, we would
need to include <time.h>. That only works, if we get the feature test
macros right. That is, define the _?_SOURCE macro as first thing.

[1] https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html

Signed-off-by: Thomas Haller <thaller@redhat.com>
---
 src/meta.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/src/meta.c b/src/meta.c
index bf2201009a8c..8508b11e70ce 100644
--- a/src/meta.c
+++ b/src/meta.c
@@ -10,6 +10,8 @@
  * Development of this code funded by Astaro AG (http://www.astaro.com/)
  */
 
+#define _GNU_SOURCE
+
 #include <errno.h>
 #include <limits.h>
 #include <stddef.h>
@@ -25,6 +27,7 @@
 #include <linux/netfilter.h>
 #include <linux/pkt_sched.h>
 #include <linux/if_packet.h>
+#include <time.h>
 
 #include <nftables.h>
 #include <expression.h>
@@ -37,10 +40,6 @@
 #include <iface.h>
 #include <json.h>
 
-#define _XOPEN_SOURCE
-#define __USE_XOPEN
-#include <time.h>
-
 static void tchandle_type_print(const struct expr *expr,
 				struct output_ctx *octx)
 {
-- 
2.41.0