Remove internal CheckCXXSymbolExists

Since Cmake 2.8.6, Cmake provides this macro.  Our use references
internal Cmake variables that have changes in Cmake 3.11.

CMakeModules/CheckCXXSymbolExists.cmake

@@ -1,42 +0,0 @@
-# - Check if a symbol exists as a function, variable, or macro in C++
-# CHECK_CXX_SYMBOL_EXISTS(<symbol> <files> <variable>)
-#
-# Check that the <symbol> is available after including given header
-# <files> and store the result in a <variable>.  Specify the list
-# of files in one argument as a semicolon-separated list.
-# CHECK_CXX_SYMBOL_EXISTS() can be used to check in C++ files, as opposed
-# to CHECK_SYMBOL_EXISTS(), which works only for C.
-#
-# If the header files define the symbol as a macro it is considered
-# available and assumed to work.  If the header files declare the
-# symbol as a function or variable then the symbol must also be
-# available for linking.  If the symbol is a type or enum value
-# it will not be recognized (consider using CheckTypeSize or
-# CheckCSourceCompiles).
-#
-# The following variables may be set before calling this macro to
-# modify the way the check is run:
-#
-#  CMAKE_REQUIRED_FLAGS = string of compile command line flags
-#  CMAKE_REQUIRED_DEFINITIONS = list of macros to define (-DFOO=bar)
-#  CMAKE_REQUIRED_INCLUDES = list of include directories
-#  CMAKE_REQUIRED_LIBRARIES = list of libraries to link
-
-#=============================================================================
-# Copyright 2003-2011 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.)
-
-include(CheckSymbolExists)
-
-macro(CHECK_CXX_SYMBOL_EXISTS SYMBOL FILES VARIABLE)
-  _CHECK_SYMBOL_EXISTS("${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckSymbolExists.cxx" "${SYMBOL}" "${FILES}" "${VARIABLE}" )
-endmacro()