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/Makefile.am b/Makefile.am
index 1ba0604..deaba0f 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -3,5 +3,8 @@
 INCLUDES = $(all_includes) -I$(top_srcdir)/include
 SUBDIRS = include src tests
 
+pkgconfigdir = $(libdir)/pkgconfig
+pkgconfig_DATA = openbsc.pc
+
 #dist-hook:
 #	rm -rf `find $(distdir) -name .svn`