[network-manager-vpnc] build: fix build with older libsecret

[Lubomir Rintel <lkundrak src gnome org>]

commit ea219af18fb97cc820e2977cba4cf20d76d6eda3
Author: Lubomir Rintel <lkundrak v3 sk>
Date:   Tue Oct 27 11:11:25 2015 +0100

    build: fix build with older libsecret

 configure.ac |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index c9cbd74..6e96313 100644
--- a/configure.ac
+++ b/configure.ac
@@ -67,7 +67,11 @@ if test x"$with_gnome" != xno; then
        GTK_CFLAGS="$GTK_CFLAGS -DGDK_VERSION_MIN_REQUIRED=GDK_VERSION_3_4"
 
        PKG_CHECK_MODULES(LIBNMA, libnma >= 1.1.0)
-       PKG_CHECK_MODULES(LIBSECRET, libsecret-1)
+       PKG_CHECK_MODULES(LIBSECRET, libsecret-1 >= 0.18, [], [
+               dnl We use the secret service API that went stable in 0.18
+               PKG_CHECK_MODULES(LIBSECRET, libsecret-unstable)
+               LIBSECRET_CFLAGS="$LIBSECRET_CFLAGS -DSECRET_API_SUBJECT_TO_CHANGE"
+       ])
 
        if test x"$with_libnm_glib" != xno; then
                PKG_CHECK_MODULES(LIBNM_GTK, libnm-gtk >= 0.9.9.0)