Added constexpr specifier where possible in CBuildConfig.

This necessitated moving some function definitions into a .inc file
that is included by buildconfig.h, as constexpr functions must be defined
in the header. This new file is added to the list of QMAKE_SUBSTITUTES.

src/blackconfig/buildconfig.inc

@@ -0,0 +1,82 @@
+/* Copyright (C) 2018
+ * swift Project Community/Contributors
+ *
+ * This file is part of swift project. It is subject to the license terms in the LICENSE file found in the top-level
+ * directory of this distribution and at http://www.swift-project.org/license.html. No part of swift project,
+ * including this file, may be copied, modified, propagated, or distributed except according to the terms
+ * contained in the LICENSE file.
+ */
+
+#ifndef IN_BUILDCONFIG_H
+#error This file is only to be included by buildconfig.h
+#endif
+
+// in-line definitions with qmake substitutions
+#include "buildconfig_gen.inc"
+
+//! \cond PRIVATE
+
+namespace BlackConfig
+{
+    constexpr bool CBuildConfig::isCompiledWithMsFlightSimulatorSupport()
+    {
+        return CBuildConfig::isCompiledWithFs9Support() || CBuildConfig::isCompiledWithFsxSupport() || CBuildConfig::isCompiledWithP3DSupport();
+    }
+
+    constexpr bool CBuildConfig::isCompiledWithFlightSimulatorSupport()
+    {
+        return CBuildConfig::isCompiledWithFsxSupport() || CBuildConfig::isCompiledWithXPlaneSupport();
+    }
+
+    constexpr bool CBuildConfig::isRunningOnWindowsNtPlatform()
+    {
+#ifdef Q_OS_WIN
+        return true;
+#else
+        return false;
+#endif
+    }
+
+    constexpr bool CBuildConfig::isRunningOnMacOSPlatform()
+    {
+#ifdef Q_OS_MACOS
+        return true;
+#else
+        return false;
+#endif
+    }
+
+    constexpr bool CBuildConfig::isRunningOnLinuxPlatform()
+    {
+#ifdef Q_OS_LINUX
+        return true;
+#else
+        return false;
+#endif
+    }
+
+    constexpr bool CBuildConfig::isRunningOnUnixPlatform()
+    {
+        return CBuildConfig::isRunningOnMacOSPlatform() || CBuildConfig::isRunningOnLinuxPlatform();
+    }
+
+    constexpr bool CBuildConfig::isDebugBuild()
+    {
+#ifdef QT_DEBUG
+        return true;
+#else
+        return false;
+#endif
+    }
+
+    constexpr bool CBuildConfig::isReleaseBuild()
+    {
+#ifdef QT_NO_DEBUG
+        return true;
+#else
+        return false;
+#endif
+    }
+} // ns
+
+//! \endcond