runtime: add windows support for nr_cpus()

vlib/runtime/runtime.v

@@ -1,16 +1,32 @@
+// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+
 module runtime
 
-/*
-$if linux {
-	#include <sys/sysinfo.h>
-	fn C.get_nprocs() int
-}
-*/
+import os
 
+//$if linux {
+fn C.sysconf(name int) i64
+//}
+
+//$if windows {
+fn C.GetCurrentProcessorNumber() u32
+//}
 
 pub fn nr_cpus() int {
-	//$if linux {
-		//return C.get_nprocs()
-	//}
-	return 0
+	$if linux {
+		return int(C.sysconf(C._SC_NPROCESSORS_ONLN))
+	}
+	$if mac {
+		return int(C.sysconf(C._SC_NPROCESSORS_ONLN))
+	}
+	$if windows {
+		mut nr := int(C.GetCurrentProcessorNumber())
+		if nr == 0 {
+			nr = os.getenv('NUMBER_OF_PROCESSORS').int()
+		}
+		return nr
+	}
+	return 1
 }