[network-manager-openvpn] build: fix linking libnm-vpn-plugin-openvpn.la for dlopen()

[Thomas Haller <thaller src gnome org>]

commit 220b38717f8d0fc16c8b9d68cfd6e5b5751e4669
Author: Dennis Schridde <devurandom gmx net>
Date:   Mon Aug 15 16:36:57 2016 +0200

    build: fix linking libnm-vpn-plugin-openvpn.la for dlopen()
    
    On some systems, dlopen() is not in libdl.so [1].
    
    [1] https://autotools.io/autoconf/finding.html
    
    https://bugzilla.gnome.org/show_bug.cgi?id=769783

 configure.ac           |    3 +++
 properties/Makefile.am |    3 ++-
 2 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 34829f4..3988bad 100644
--- a/configure.ac
+++ b/configure.ac
@@ -38,6 +38,9 @@ dnl
 AC_PROG_GCC_TRADITIONAL
 AC_FUNC_MEMCMP
 AC_CHECK_FUNCS(select socket uname)
+AC_SEARCH_LIBS([dlopen], [dl dld], [], [
+  AC_MSG_ERROR([unable to find the dlopen() function])
+])
 
 dnl ensure that when the Automake generated makefile calls aclocal,
 dnl it honours the $ACLOCAL_FLAGS environment variable
diff --git a/properties/Makefile.am b/properties/Makefile.am
index 552e537..04ed5cd 100644
--- a/properties/Makefile.am
+++ b/properties/Makefile.am
@@ -57,7 +57,8 @@ libnm_vpn_plugin_openvpn_la_CFLAGS =                    \
         $(LIBNM_CFLAGS)
 
 libnm_vpn_plugin_openvpn_la_LIBADD = \
-        $(LIBNM_LIBS)
+        $(LIBNM_LIBS) \
+        $(DLOPEN)
 
 libnm_vpn_plugin_openvpn_la_LDFLAGS = \
         -avoid-version \