Create Linux GNU compiler flag consolidation macro

This macro will be used for GNU compiler flags that are specific to
Linux but not to any language.

Modules/Platform/Linux-GNU-C.cmake

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+INCLUDE(Platform/Linux-GNU)
+__linux_compiler_gnu(C)

Modules/Platform/Linux-GNU-CXX.cmake

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+INCLUDE(Platform/Linux-GNU)
+__linux_compiler_gnu(CXX)

Modules/Platform/Linux-GNU-Fortran.cmake

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+INCLUDE(Platform/Linux-GNU)
+__linux_compiler_gnu(Fortran)

Modules/Platform/Linux-GNU.cmake

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+
+#=============================================================================
+# Copyright 2010 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 distributed 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(__LINUX_COMPILER_GNU)
+  return()
+endif()
+set(__LINUX_COMPILER_GNU 1)
+
+macro(__linux_compiler_gnu lang)
+endmacro()