CheckIncludeFiles: Use modern C declaration for 'main' (#15058)

Otherwise some compilers will incorrectly fail the checks when certain
warning/error flags are enabled.

Modules/CheckIncludeFiles.cmake

@@ -53,7 +53,7 @@ macro(CHECK_INCLUDE_FILES INCLUDE VARIABLE)
         "${CMAKE_CONFIGURABLE_FILE_CONTENT}#include <${FILE}>\n")
     endforeach()
     set(CMAKE_CONFIGURABLE_FILE_CONTENT
-      "${CMAKE_CONFIGURABLE_FILE_CONTENT}\n\nint main(){return 0;}\n")
+      "${CMAKE_CONFIGURABLE_FILE_CONTENT}\n\nint main(void){return 0;}\n")
     configure_file("${CMAKE_ROOT}/Modules/CMakeConfigurableFile.in"
       "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/CheckIncludeFiles.c" @ONLY)