OS X: Add platform-specific Clang compiler info files (#13536)

Clang has the same interface as GNU except that we do not need to test
for the deployment target and sysroot flags.  Simply set variables

 CMAKE_${lang}_HAS_ISYSROOT
 CMAKE_${lang}_OSX_DEPLOYMENT_TARGET_FLAG

to true because every version of Clang available on OS X supports these
flags.

Modules/Platform/Darwin-Clang-C.cmake

@@ -0,0 +1,2 @@
+include(Platform/Darwin-Clang)
+__darwin_compiler_clang(C)

Modules/Platform/Darwin-Clang-CXX.cmake

@@ -0,0 +1,2 @@
+include(Platform/Darwin-Clang)
+__darwin_compiler_clang(CXX)

Modules/Platform/Darwin-Clang.cmake

@@ -0,0 +1,26 @@
+
+#=============================================================================
+# Copyright 2002-2012 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 module is shared by multiple languages; use include blocker.
+if(__DARWIN_COMPILER_CLANG)
+  return()
+endif()
+set(__DARWIN_COMPILER_CLANG 1)
+
+macro(__darwin_compiler_clang lang)
+  set(CMAKE_SHARED_LIBRARY_CREATE_${lang}_FLAGS "-dynamiclib -Wl,-headerpad_max_install_names")
+  set(CMAKE_SHARED_MODULE_CREATE_${lang}_FLAGS "-bundle -Wl,-headerpad_max_install_names")
+  set(CMAKE_${lang}_OSX_DEPLOYMENT_TARGET_FLAG "-mmacosx-version-min=")
+  set(CMAKE_${lang}_HAS_ISYSROOT 1)
+endmacro()