CheckSymbolExists: Rename implementation macro

Currently the implementation macro for `check_symbol_exists` is
`_check_symbol_exists`, which impedes CMake users from overriding the
`check_symbol_exists` macro themselves.

Modules/CheckCXXSymbolExists.cmake

@@ -35,5 +35,5 @@
 include(CheckSymbolExists)
 
 macro(CHECK_CXX_SYMBOL_EXISTS SYMBOL FILES VARIABLE)
-  _CHECK_SYMBOL_EXISTS("${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckSymbolExists.cxx" "${SYMBOL}" "${FILES}" "${VARIABLE}" )
+  __CHECK_SYMBOL_EXISTS_IMPL("${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckSymbolExists.cxx" "${SYMBOL}" "${FILES}" "${VARIABLE}" )
 endmacro()

Modules/CheckSymbolExists.cmake

@@ -45,15 +45,15 @@ the way the check is run:
 
 macro(CHECK_SYMBOL_EXISTS SYMBOL FILES VARIABLE)
   if(CMAKE_C_COMPILER_LOADED)
-    _CHECK_SYMBOL_EXISTS("${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckSymbolExists.c" "${SYMBOL}" "${FILES}" "${VARIABLE}" )
+    __CHECK_SYMBOL_EXISTS_IMPL("${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckSymbolExists.c" "${SYMBOL}" "${FILES}" "${VARIABLE}" )
   elseif(CMAKE_CXX_COMPILER_LOADED)
-    _CHECK_SYMBOL_EXISTS("${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckSymbolExists.cxx" "${SYMBOL}" "${FILES}" "${VARIABLE}" )
+    __CHECK_SYMBOL_EXISTS_IMPL("${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckSymbolExists.cxx" "${SYMBOL}" "${FILES}" "${VARIABLE}" )
   else()
     message(FATAL_ERROR "CHECK_SYMBOL_EXISTS needs either C or CXX language enabled")
   endif()
 endmacro()
 
-macro(_CHECK_SYMBOL_EXISTS SOURCEFILE SYMBOL FILES VARIABLE)
+macro(__CHECK_SYMBOL_EXISTS_IMPL SOURCEFILE SYMBOL FILES VARIABLE)
   if(NOT DEFINED "${VARIABLE}" OR "x${${VARIABLE}}" STREQUAL "x${VARIABLE}")
     set(CMAKE_CONFIGURABLE_FILE_CONTENT "/* */\n")
     set(MACRO_CHECK_SYMBOL_EXISTS_FLAGS ${CMAKE_REQUIRED_FLAGS})