re-introduce optional build of DAHDI input plugin
We should only build it when the dahdi/user.h file is actually present.
diff --git a/configure.ac b/configure.ac
index 3fdc050..f14f047 100644
--- a/configure.ac
+++ b/configure.ac
@@ -38,7 +38,7 @@
PKG_CHECK_MODULES(LIBOSMOVTY, libosmovty >= 0.3.0)
PKG_CHECK_MODULES(LIBOSMOGSM, libosmogsm >= 0.3.0)
-AC_CHECK_HEADERS(dahdi/user.h,,AC_MSG_ERROR(DAHDI input driver headers missing))
+AC_CHECK_HEADERS(dahdi/user.h,,AC_MSG_WARN(DAHDI input driver will not be built))
AC_OUTPUT(
libosmoabis.pc