[network-manager-pptp] build: fix linking against libdl.so for dlopen()

[Thomas Haller <thaller src gnome org>]

commit f3e916714196d7687bb8920a9d0339f7fe4bbf7c
Author: Thomas Haller <thaller redhat com>
Date:   Wed Sep 7 18:28:57 2016 +0200

    build: fix linking against libdl.so for dlopen()

 configure.ac           |    3 +++
 properties/Makefile.am |    4 +++-
 2 files changed, 6 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index c8c7c83..1a83922 100644
--- a/configure.ac
+++ b/configure.ac
@@ -51,6 +51,9 @@ AC_PROG_GCC_TRADITIONAL
 AC_FUNC_MEMCMP
 AC_CHECK_FUNCS(select socket uname)
 
+AC_SEARCH_LIBS([dlopen], [dl dld], [], [ac_cv_search_dlopen=])
+AC_SUBST([DL_LIBS], "$ac_cv_search_dlopen")
+
 dnl
 dnl GNOME support
 dnl
diff --git a/properties/Makefile.am b/properties/Makefile.am
index 625cf06..1a3e750 100644
--- a/properties/Makefile.am
+++ b/properties/Makefile.am
@@ -45,7 +45,9 @@ libnm_vpn_plugin_pptp_la_CFLAGS = \
     $(LIBNM_CFLAGS)
 
 libnm_vpn_plugin_pptp_la_LIBADD = \
-    $(LIBNM_LIBS)
+    $(LIBNM_LIBS) \
+    $(LD_LIBS) \
+    $(NULL)
 
 libnm_vpn_plugin_pptp_la_LDFLAGS = \
     -avoid-version \