title: how can I check a particular gcc feature in configure.ac
body: <p>For example, gcc 4.7 has a new feature -Wnarrowing. In configure.ac, how can I test where a feature is supported by the current gcc or not?<br>
  There's a <a href="http://git.savannah.gnu.org/cgit/gnulib.git/tree/m4/manywarnings.m4" rel="noreferrer">file</a> in gnulibc, but doesn't make much sense to me.</p>