title: While installing on OSX Sierra via gcc-6, keep having "FATAL:/opt/local/bin/../libexec/as/x86_64/as: I don't understand 'm' flag!" error
body: <h2>Environment info</h2>
  
  <p>Operating System:
  macOS 10.12.2 (16C68)</p>
  
  <p>Compiler:
  gcc-6</p>
  
  <h2>Steps to reproduce</h2>
  
  <p>I've installed gcc-6 and modified config.mk as required into
  <code>
  export CC = gcc-6
  export CXX = g++-6
  </code>
  But keep having this error:
  <code>
  g++-6 -c -std=c++0x -Wall -Wno-unknown-pragmas -Iinclude   -Idmlc-core/include -Irabit/include -O3 -funroll-loops -msse2 -fPIC -fopenmp src/learner.cc -o build/learner.o
  FATAL:/opt/local/bin/../libexec/as/x86_64/as: I don't understand 'm' flag!
  </code></p>
  
  <h2>What have you tried?</h2>
  
  <ol>
  <li>Reinstall <code>XCode</code></li>
  <li>Reinstall <code>gcc</code></li>
  <li>Run <code>make clean_all &amp;&amp; make -4j</code></li>
  </ol>
  
  <p>But still went wrong. Any idea?</p>