[NetworkManager-pptp] build: enable -D_GNU_SOURCE

[Lubomir Rintel <lkundrak src gnome org>]

commit 6705863c7b8ae5ae737b0dbfe0abb1fad5272464
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 d42fba3..55564ed 100644
--- a/configure.ac
+++ b/configure.ac
@@ -21,6 +21,8 @@ AC_PROG_INSTALL
 AC_PROG_LIBTOOL
 AC_PATH_PROG(GLIB_COMPILE_RESOURCES, glib-compile-resources)
 
+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.])