glcpp: Avoid warnings in generated flex code.

glcpp: Avoid warnings in generated flex code.

We define the YY_NO_INPUT macro to avoid one needless function being
generated.

for the other needless functions, (yyunput and yy_top_state), we add a
new UNREACHABLE start condition and call these functions from an
action there. This doesn't change functionality at all, (since we
never enter the UNREACHABLE start condition), but makes the compiler
stop complaining about these two functions being defined but not used.