Enable DAHDI support by default; require --disable-dahdi otherwise

libosmo-abis was built with DAHDI support, if the related header files
were present at built time, and without if not.  This kind of automagic
enabling/disabling of features is wrong.  Let's require DAHDI support by
default, and force the user to take a conscious decision by using an
explicit --disable-dahdi if he doesn't want it.

At the same time, update debian/control to list dahdi-source as build
dependency.

Change-Id: Id9f7f063e7ca9e3ab4aa96fc93f243caf50fb66a
Closes: OS#4248
diff --git a/debian/control b/debian/control
index 24a082e..b9e2078 100644
--- a/debian/control
+++ b/debian/control
@@ -6,6 +6,7 @@
                autotools-dev,
                autoconf,
                automake,
+               dahdi-source,
                libtool,
                dh-autoreconf,
                libdpkg-perl,