display selected C++ standard as part of the CMake summary output

cmake/CMakeLists.txt

@@ -940,6 +940,7 @@ message(STATUS "<<< Compilers and Flags: >>>
 -- C++ Compiler:     ${CMAKE_CXX_COMPILER}
       Type:          ${CMAKE_CXX_COMPILER_ID}
       Version:       ${CMAKE_CXX_COMPILER_VERSION}
+      C++ Standard:  ${CMAKE_CXX_STANDARD}
       C++ Flags:    ${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_${BTYPE}}
       Defines:       ${DEFINES}")
 get_target_property(OPTIONS lammps COMPILE_OPTIONS)