message: added support for linux Bison(2.3) generated code in 'generate_flex.py' script
diff: import os
  import subprocess
  import re
  +from sys import platform
  from python_helpers import open_utf8
  pg_path = os.path.join('third_party', 'libpg_query')
  @@ -40,7 +41,10 @@ text = re.sub('[(]void[)][ ]*fprintf', '//', text)
  text = re.sub('exit[(]', 'throw std::runtime_error(msg); //', text)
  text = re.sub(r'\n\s*if\s*[(]\s*!\s*yyin\s*[)]\s*\n\s*yyin\s*=\s*stdin;\s*\n', '\n', text)
  text = re.sub(r'\n\s*if\s*[(]\s*!\s*yyout\s*[)]\s*\n\s*yyout\s*=\s*stdout;\s*\n', '\n', text)
  -text = re.sub(r'[#]ifdef\s*YY_STDINIT\n\s*yyin = stdin;\n\s*yyout = stdout;\n[#]else\n\s*yyin = [(]FILE [*][)] 0;\n\s*yyout = [(]FILE [*][)] 0;\n[#]endif', ' yyin = (FILE *) 0;\n yyout = (FILE *) 0;', text)
  +
  +file_null = 'NULL' if platform == 'linux' else '[(]FILE [*][)] 0'
  +
  +text = re.sub(rf'[#]ifdef\s*YY_STDINIT\n\s*yyin = stdin;\n\s*yyout = stdout;\n[#]else\n\s*yyin = {file_null};\n\s*yyout = {file_null};\n[#]endif', ' yyin = (FILE *) 0;\n yyout = (FILE *) 0;', text)
  if 'stdin;' in text:
  print("STDIN not removed!")
  