Default to available cpu threads for -j option.

This adds a `b2::system_info` class to obtain available information on
system we are running in. Currently provides CPU counts.
And currently only implemented for macOS.

src/engine/sysinfo.cpp

@@ -0,0 +1,62 @@
+/*  Copyright 2019 Rene Rivera
+ *  Distributed under the Boost Software License, Version 1.0.
+ *  (See accompanying file LICENSE_1_0.txt or http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#include "sysinfo.h"
+#include "jam.h"
+#include "output.h"
+
+#ifdef OS_MACOSX
+#include <sys/types.h>
+#include <sys/sysctl.h>
+#endif
+
+
+b2::system_info::system_info()
+{
+}
+
+unsigned int b2::system_info::cpu_core_count()
+{
+    if (cpu_core_count_ == 0)
+    {
+        #ifdef OS_MACOSX
+        int out_hw_ncpu = 0;
+        size_t len_hw_ncpu = sizeof(out_hw_ncpu);
+        int result = ::sysctlbyname(
+            "hw.physicalcpu", &out_hw_ncpu, &len_hw_ncpu, nullptr, 0);
+        if (result == 0)
+        {
+            cpu_core_count_ = out_hw_ncpu;
+        }
+        #endif
+        if (cpu_core_count_ == 0)
+        {
+            cpu_core_count_ = 1;
+        }
+    }
+    return cpu_core_count_;
+}
+
+unsigned int b2::system_info::cpu_thread_count()
+{
+    if (cpu_thread_count_ == 0)
+    {
+        #ifdef OS_MACOSX
+        int out_hw_ncpu = 0;
+        size_t len_hw_ncpu = sizeof(out_hw_ncpu);
+        int result = ::sysctlbyname(
+            "hw.logicalcpu", &out_hw_ncpu, &len_hw_ncpu, nullptr, 0);
+        if (result == 0)
+        {
+            cpu_thread_count_ = out_hw_ncpu;
+        }
+        #endif
+        if (cpu_thread_count_ == 0)
+        {
+            cpu_thread_count_ = cpu_core_count();
+        }
+    }
+    return cpu_thread_count_;
+}