libosmocore libusb integration

Osmocom applications typically use libosmocore select.[ch] event loop
code as their main event dispatch mechanism.  When they want to deal
with libusb in a non-blocking/asynchronous way, they need to integrate
libusb into that select().

The new libosmousb is doing exactly that: Providing a shared utility
library for Osmocom programs that wish to use libusb.  This is useful
for example in simtrace2 host utilitie as well as osmo-e1d.

Change-Id: I656a1a38cbb5b1f3a9145d2869d3b4d0adefcae3
Closes: OS#4299
diff --git a/Makefile.am b/Makefile.am
index 7b6ba0a..7390f98 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,11 +1,12 @@
 ACLOCAL_AMFLAGS = -I m4
 
 AM_CPPFLAGS = $(all_includes) -I$(top_srcdir)/include
-SUBDIRS = include src src/vty src/codec src/gsm src/coding src/gb src/ctrl src/sim src/pseudotalloc utils tests
+SUBDIRS = include src src/vty src/codec src/gsm src/coding src/gb src/ctrl src/sim src/pseudotalloc src/usb utils tests
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libosmocore.pc libosmocodec.pc libosmovty.pc libosmogsm.pc \
-		 libosmogb.pc libosmoctrl.pc libosmocoding.pc libosmosim.pc
+		 libosmogb.pc libosmoctrl.pc libosmocoding.pc libosmosim.pc \
+		 libosmousb.pc
 
 aclocaldir = $(datadir)/aclocal
 dist_aclocal_DATA = m4/osmo_ac_code_coverage.m4 \