core: use libnl CFLAGS and LIBS consistently

This patch is used in OpenEmbedded [1] since commit 154bd72b [2][3].

[1] http://www.openembedded.org/
[2] http://cgit.openembedded.org/cgit.cgi/openembedded/log/recipes/networkmanager/networkmanager/0001-respect-libnl-flags-also-in-backends-ip6-manager.patch
[3] http://cgit.openembedded.org/cgit.cgi/openembedded/commit/?id=154bd72b1ca859afcae5ed4fe99ed611a13a475c

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>

src/backends/Makefile.am

@@ -63,10 +63,12 @@ endif
 
 libnmbackend_la_LIBADD += \
 	$(top_builddir)/src/logging/libnm-logging.la \
+	$(LIBNL_LIBS) \
 	$(DBUS_LIBS) \
 	$(GLIB_LIBS)
 
 libnmbackend_la_CPPFLAGS = \
+	$(LIBNL_CFLAGS) \
 	$(DBUS_CFLAGS) \
 	$(GLIB_CFLAGS) \
 	-DG_DISABLE_DEPRECATED \

src/ip6-manager/Makefile.am

@@ -13,6 +13,7 @@ libip6_manager_la_SOURCES = \
 	nm-ip6-manager.h
 
 libip6_manager_la_CPPFLAGS = \
+	$(LIBNL_CFLAGS) \
 	$(DBUS_CFLAGS) \
 	$(GLIB_CFLAGS) \
 	-DG_DISABLE_DEPRECATED
@@ -20,6 +21,7 @@ libip6_manager_la_CPPFLAGS = \
 libip6_manager_la_LIBADD = \
 	$(top_builddir)/marshallers/libmarshallers.la \
 	$(top_builddir)/src/logging/libnm-logging.la \
+	$(LIBNL_LIBS) \
 	$(DBUS_LIBS) \
 	$(GLIB_LIBS)