title: intel compilers, silence commandline warnings
body: <p>I just started building some code with the intel c compiler -- <code>icc</code>.  Our configure script likes to add the <code>-ffast-math</code> flag and maybe a couple others which seem to be GCC specific.  Invoking <code>icc</code> with <code>-ffast-math</code> produces the following warning which I would like to silence: </p>
  
  <pre><code>icc: command line warning #10006: ignoring unknown option '-ffast-math'
  </code></pre>
  
  <p>As far as I see it, there are 2 ways it could be silenced (But I'd love to see other solutions).  First, I could turn that warning into an error which would tell configure that <code>-ffast-math</code> isn't a valid option.  I would hope that when configure tries to add that to the commandline, it would then see it isn't able to and decide that maybe adding it was a bad idea after all ... The second option (which I don't think is quite as clean) is to just tell <code>icc</code> to silence that kind of warning ...</p>
  
  <p>Responding to the comments, here's the relevant portion of <code>configure.ac</code>:</p>
  
  <pre><code># add -ffast-math etc if possible
  AX_CHECK_COMPILER_FLAGS([-ffast-math],
    [CFLAGS="$CFLAGS -ffast-math"
  ])
  
  AX_CHECK_COMPILER_FLAGS([-mtune=native -march=native],
    [CFLAGS="$CFLAGS -mtune=native -march=native"
  ])
  </code></pre>
  
  <p>That m4 macro appears to have been taken from <a href="https://github.com/alip/afprint/blob/master/m4/ax_check_compiler_flags.m4" rel="nofollow">here</a></p>
  
  <p>I suppose that fixing that to be smarter would be the "holy-grail" -- But as <code>icc</code> returns a successful exit status even when <code>-ffast-math</code> is passed (or <code>-mtune=native</code> etc.), I don't really think that there is too much that can be done there (feel free to prove me wrong).  that said, <strong>I don't want to hard-code checks for intel into the configure script...</strong>.  That seems overly messy.</p>