Use correct macro for getaddrinfo in glibc >= 2.22

Makefile

-CFLAGS += -std=c11 -D_POSIX_SOURCE -Wall -Wconversion -fPIC
+CFLAGS += -std=c11 -D_POSIX_C_SOURCE=201112 -Wall -Wconversion -fPIC
 
 goatherd: goatherd.go
 	go build goatherd.go