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/configure.in b/configure.in
index 69fa1b5..7bc8b63 100644
--- a/configure.in
+++ b/configure.in
@@ -19,6 +19,7 @@
 dnl Checks for typedefs, structures and compiler characteristics
 
 AC_OUTPUT(
+    openbsc.pc
     include/openbsc/Makefile
     include/vty/Makefile
     include/Makefile