Add platform-specific initialization step when enabling languages

Create a Modules/CMakeSystemSpecificInitialize.cmake module loaded after
CMakeSystem.cmake but before per-language initialization.  Use it to
load an optional Platform/<os>-Initialize.cmake module.  This will be
useful to do per-platform initialization that does not depend on the
language and use the results when enabling specific languages.

Modules/CMakeSystemSpecificInitialize.cmake

@@ -0,0 +1,20 @@
+
+#=============================================================================
+# Copyright 2002-2014 Kitware, Inc.
+#
+# Distributed under the OSI-approved BSD License (the "License");
+# see accompanying file Copyright.txt for details.
+#
+# This software is distributed WITHOUT ANY WARRANTY; without even the
+# implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+# See the License for more information.
+#=============================================================================
+# (To distribute this file outside of CMake, substitute the full
+#  License text for the above reference.)
+
+# This file is included by cmGlobalGenerator::EnableLanguage.
+# It is included before the compiler has been determined.
+
+include(Platform/${CMAKE_SYSTEM_NAME}-Initialize OPTIONAL)
+
+set(CMAKE_SYSTEM_SPECIFIC_INITIALIZE_LOADED 1)