pkg-config: Create and install a pkg-config file

Let automake create the openbsc.pc and then install it
into the pkg-config directory of ${libdir}. This is done
in preparation for the public API that will be added soon.

The advertised include directory will be ${incluedir}/openbsc-1.0
to follow userspace practive from glib/gtk/cairo/freetype2 and
many more.
diff --git a/openbsc.pc.in b/openbsc.pc.in
new file mode 100644
index 0000000..0d00831
--- /dev/null
+++ b/openbsc.pc.in
@@ -0,0 +1,11 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@/openbsc-1.0
+
+Name: OpenBSC
+Description: OpenBSC base station controller
+Requires:
+Version: @VERSION@
+Libs: -L${libdir} -lopenbsc
+Cflags: -I${includedir}