[PATCH nft 4/6] configure: use AC_USE_SYSTEM_EXTENSIONS to get _GNU_SOURCE

[Thomas Haller <thaller@redhat.com>]

Let "configure" detect which features are available. Also, nftables is a
Linux project, so portability beyond gcc/clang and glibc/musl is less
relevant. And even if it were, then feature detection by "configure"
would still be preferable.

Use AC_USE_SYSTEM_EXTENSIONS ([1]).

Apparently available since autoconf 2.60, from 2006 ([2]).

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.67/html_node/Posix-Variants.html#index-AC_005fUSE_005fSYSTEM_005fEXTENSIONS-1046
[2] https://lists.gnu.org/archive/html/autoconf/2006-06/msg00111.html

Signed-off-by: Thomas Haller <thaller@redhat.com>
---
 configure.ac         | 3 +++
 include/nftdefault.h | 2 --
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index 42f0dc4cf392..9d859307adaa 100644
--- a/configure.ac
+++ b/configure.ac
@@ -45,6 +45,9 @@ fi
 AM_PROG_AR
 LT_INIT([disable-static])
 AM_PROG_CC_C_O
+
+AC_USE_SYSTEM_EXTENSIONS
+
 AC_EXEEXT
 CHECK_GCC_FVISIBILITY
 
diff --git a/include/nftdefault.h b/include/nftdefault.h
index b7ebad7fddc0..0912cd188850 100644
--- a/include/nftdefault.h
+++ b/include/nftdefault.h
@@ -2,8 +2,6 @@
 #ifndef NFTABLES_NFTDEFAULT_H
 #define NFTABLES_NFTDEFAULT_H
 
-#define _GNU_SOURCE
-
 #include <config.h>
 
 #endif /* NFTABLES_NFTDEFAULT_H */
-- 
2.41.0