build: please Clang++ 3.2+ on Flex scanners

build: please Clang++ 3.2+ on Flex scanners

Clang++, with -Wall, rejects code generated by Flex (for C scanners):

  CXX      examples/calc++/examples_calc___calc__-calc++-scanner.o
  In file included from examples/calc++/calc++-scanner.cc:1:
  error: implicit conversion of NULL constant to 'bool' [-Werror,-Wnull-conversion]
  if ( ! ( (yy_buffer_stack) ? (yy_buffer_stack)[(yy_buffer_stack_top)] : __null) ) {
       ~                                                                  ^~~~~~
                                                                          false
* configure.ac (WARN_NO_NULL_CONVERSION_CXXFLAGS): Compute it.
* examples/calc++/local.mk (examples_calc___calc___CXXFLAGS): Use it.