update_engine: Split DBus client proxies to a library.

In order to allow other daemons call update_engine's DBus interface, we
split the generated interface to its own client library.

BUG=b:23084607
TEST=rm -rf /build/storm/var/cache/portage/chromeos-base/update_engine/out/Default/ ; emerge-link update_engine

Change-Id: Iac09b3269d9ffab599b27888b4a1a2824d262059
Reviewed-on: https://chromium-review.googlesource.com/294064
Commit-Queue: Alex Deymo <deymo@chromium.org>
Trybot-Ready: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
diff --git a/libupdate_engine-client.pc.in b/libupdate_engine-client.pc.in
new file mode 100644
index 0000000..4c87e1d
--- /dev/null
+++ b/libupdate_engine-client.pc.in
@@ -0,0 +1,6 @@
+include_dir=@INCLUDE_DIR@
+
+Name: libupdate_engine-client
+Version: 1.0
+Description: update_engine client interface library
+Cflags: -I${include_dir}