sabp: Generate C/H files for SABP; create libosmo-sabp

This uses the (modified) Osmocom asn1c on the (modified) SABP ASN.1
syntax to generate C code + header files for SABP parsing/encoding.

It also adds some helper code for message encoding and decoding as well
as a new libosmo-sabp shared library which can be used by programs to
implement SABP related functionality.

Change-Id: Ib9580d1af96354398da4c9f97b28a0e23d56e275
diff --git a/Makefile.am b/Makefile.am
index 22dcfe4..40ea321 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -3,7 +3,7 @@
 SUBDIRS = src include doc contrib
 
 pkgconfigdir = $(libdir)/pkgconfig
-pkgconfig_DATA = libosmo-ranap.pc
+pkgconfig_DATA = libosmo-ranap.pc libosmo-sabp.pc
 
 EXTRA_DIST = asn1 .version README.md