Fixing issue #15302 -- lwip doesn't support signals (#15303)

* Fixing issue #15302 -- lwip doesn't support signals

* Adding test to catch issue #15302 -- lwip/freertos net library
don't try to build / run on windows, it'll compile only but not run

Fixing issue #15302 -- reworking test to compile on other platforms

tests/stdlib/tlwip.nim

@@ -0,0 +1,30 @@
+discard """
+  targets: "c"
+  cmd: "nim $target --os:freertos --gc:arc $options $file"
+  disabled: "bsd"
+  disabled: "windows"
+  action: compile
+"""
+
+# Note:
+#   This file tests FreeRTOS/LwIP cross-compilation on UNIX platforms
+#   Windows should run when compiled with esp-idf, however I'm not
+#   sure how to test for only compilation on Windows without running 
+#   a test exe
+# 
+# Note:
+#   disabling *BSDs since they're not playing well with `gcc`
+
+import net
+import asynchttpserver, asyncdispatch
+
+proc cb*(req: Request) {.async.} =
+  await req.respond(Http200, "Hello World")
+
+proc run_http_server*() {.exportc.} =
+  echo "starting http server"
+  var server = newAsyncHttpServer()
+
+  waitFor server.serve(Port(8181), cb)
+
+echo("ok")