nix-daemon: Use a thread instead of SIGPOLL to catch client disconnects

The thread calls poll() to wait until a HUP (or other error event)
happens on the client connection. If so, it sends SIGINT to the main
thread, which is then cleaned up normally. This is much nicer than
messing around with SIGPOLL.

local.mk

@@ -5,6 +5,7 @@ endif
 dist-files += configure config.h.in nix.spec
 
 GLOBAL_CXXFLAGS += -I . -I src -I src/libutil -I src/libstore -I src/libmain -I src/libexpr
+GLOBAL_LDFLAGS += -pthread
 
 $(foreach i, config.h $(call rwildcard, src/lib*, *.hh), $(eval $(call install-file-in, $(i), $(includedir)/nix, 0644)))