e1d: Initial osmo-e1d support

osmo-e1d is part of the Osmocom 'software defined E1 interface,
which consists of a USB device for the actual E1 hardware interfacing,
and a daemon (osmo-e1d) implementing a libusb-based driver.

This commit adds initial support for talking to osmo-e1d using
the related libosmoe1d library.  You need to use '--enable-e1d'
at configure time to enable it.

Change-Id: Ia0431c124e3b5b4108aee7b109d8c4bb0d8b45d4
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/configure.ac b/configure.ac
index 5a83eb1..35b8328 100644
--- a/configure.ac
+++ b/configure.ac
@@ -75,6 +75,27 @@
 	AC_MSG_WARN([DAHDI input driver will not be built])
 fi
 
+AC_ARG_ENABLE([e1d],
+	[AS_HELP_STRING(
+		[--enable-e1d],
+		[Enable osmo-e1d driver support]
+	)],
+	[
+		ENABLE_E1D=$enableval
+	],
+	[
+		ENABLE_E1D="no"
+	])
+AS_IF([test "x$ENABLE_E1D" = "xyes"], [
+	PKG_CHECK_MODULES(LIBOSMOE1D, libosmo-e1d)
+])
+AM_CONDITIONAL(ENABLE_E1D, test "x$ENABLE_E1D" = "xyes")
+AC_SUBST(ENABLE_E1D)
+if test x"$ENABLE_E1D" = x"yes"
+then
+        AC_DEFINE([HAVE_E1D], [1], [Enable osmo-e1d driver support])
+fi
+
 AC_ARG_ENABLE(sanitize,
 	[AS_HELP_STRING(
 		[--enable-sanitize],