build: Detecting if UNIX socket is available and disabling it if it is not

Change-Id: Iacdffe36223c085136a4fe09587ce304a3029853
diff --git a/daemon/face/unix-stream-face.hpp b/daemon/face/unix-stream-face.hpp
index c04bec2..cc97c21 100644
--- a/daemon/face/unix-stream-face.hpp
+++ b/daemon/face/unix-stream-face.hpp
@@ -9,6 +9,10 @@
 
 #include "stream-face.hpp"
 
+#ifndef HAVE_UNIX_SOCKETS
+#error "Cannot include this file when UNIX sockets are not available"
+#endif
+
 namespace nfd
 {