Use '-o -' instead of '-o /bin/cat'.

c++/src/capnp/compiler/capnp.c++

@@ -245,9 +245,8 @@ public:
                              "to use.  If <lang> is a simple word, the compiler searches for a plugin "
                              "called 'capnpc-<lang>' in $PATH.  If <lang> is a file path "
                              "containing slashes, it is interpreted as the exact plugin "
-                             "executable file name, and $PATH is not searched.  The special case of "
-                             "'/bin/cat' is hardcoded to dump the request to standard output, as "
-                             "this is a common operation and /bin/cat is not available on all systems.")
+                             "executable file name, and $PATH is not searched.  If <lang> is '-', "
+                             "the compiler dumps the request to standard output.")
            .addOptionWithArg({"src-prefix"}, KJ_BIND_METHOD(*this, addSourcePrefix), "<prefix>",
                              "If a file specified for compilation starts with <prefix>, remove "
                              "the prefix for the purpose of deciding the names of output files.  "
@@ -442,7 +441,7 @@ public:
     }
 
     for (auto& output: outputs) {
-      if (kj::str(output.name) == "/bin/cat") {
+      if (kj::str(output.name) == "-") {
         writeMessageToFd(STDOUT_FILENO, message);
         continue;
       }