Bazel qnx (#1192)

* Don't link pthread on QNX

On QNX, pthread is part of libc [1]. There's no separate pthread library
to link.

[1] https://www.qnx.com/developers/docs/7.1/index.html#com.qnx.doc.neutrino.lib_ref/topic/p/pthread_create.html

* Explain that QNX doesn't have pthread lib

BUILD.bazel

@@ -2,8 +2,18 @@ load("@rules_cc//cc:defs.bzl", "cc_library")
 
 licenses(["notice"])
 
+config_setting(
+    name = "qnx",
+    constraint_values = ["@platforms//os:qnx"],
+    values = {
+        "cpu": "x64_qnx",
+    },
+    visibility = [":__subpackages__"],
+)
+
 config_setting(
     name = "windows",
+    constraint_values = ["@platforms//os:windows"],
     values = {
         "cpu": "x64_windows",
     },

README.md

@@ -227,6 +227,11 @@ can link to pthread by adding `-pthread` to your linker command. Note, you can
 also use `-lpthread`, but there are potential issues with ordering of command
 line parameters if you use that.
 
+On QNX, the pthread library is part of libc and usually included automatically
+(see
+[`pthread_create()`](https://www.qnx.com/developers/docs/7.1/index.html#com.qnx.doc.neutrino.lib_ref/topic/p/pthread_create.html)).
+There's no separate pthread library to link.
+
 ### Building with Visual Studio 2015 or 2017
 
 The `shlwapi` library (`-lshlwapi`) is required to support a call to `CPUInfo` which reads the registry. Either add `shlwapi.lib` under `[ Configuration Properties > Linker > Input ]`, or use the following: