[NetworkManager-vpnc] build: enable -D_GNU_SOURCE

[Lubomir Rintel <lkundrak src gnome org>]

commit 463ff3db6452a56c8eb2b967068450af6b8707d1
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.

 configure.ac | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/configure.ac b/configure.ac
index 272c4c2..b31a156 100644
--- a/configure.ac
+++ b/configure.ac
@@ -19,6 +19,8 @@ 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.])