[NetworkManager-openconnect] build: enable -D_GNU_SOURCE

[Lubomir Rintel <lkundrak src gnome org>]

commit c9b9b0e52bc5717b3937eca8853b35dac1c916c5
Author: Lubomir Rintel <lkundrak v3 sk>
Date:   Wed Oct 24 15:23:47 2018 +0200

    build: enable -D_GNU_SOURCE
    
    The shared files are GNU sources: they use glibc dynamic linker
    specialties.

 auth-dialog/main.c | 1 -
 configure.ac       | 4 ++++
 2 files changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/auth-dialog/main.c b/auth-dialog/main.c
index 585d072..5c1518e 100644
--- a/auth-dialog/main.c
+++ b/auth-dialog/main.c
@@ -28,7 +28,6 @@
 #include <string.h>
 #include <errno.h>
 #include <unistd.h>
-#define _GNU_SOURCE
 #include <getopt.h>
 
 #include <libxml/parser.h>
diff --git a/configure.ac b/configure.ac
index e919fe1..5dc9b79 100644
--- a/configure.ac
+++ b/configure.ac
@@ -11,6 +11,8 @@ AC_CONFIG_MACRO_DIR([m4])
 
 AC_CONFIG_HEADERS([config.h])
 
+AC_USE_SYSTEM_EXTENSIONS
+
 dnl
 dnl Require programs
 dnl
@@ -19,6 +21,8 @@ AM_PROG_CC_C_O
 AC_PROG_INSTALL
 AC_PROG_LIBTOOL
 
+AC_GNU_SOURCE
+
 AC_CHECK_PROG([has_file], file, yes, no)
 if test x$has_file = xno ; then
     AC_MSG_ERROR(["file" utility not found.])