title: "ld: Warning: directory not found" when attempting to install R package
body: <p>I'm currently on a Mac running El Cap 10.11.6 attempting to install the 'robustbase' package in R Studio and am encountering the following error:</p>
  
  <pre><code>    ld: warning: directory not found for option '-L/usr/local/gfortran/lib/gcc/x86_64-apple-darwin15/6.1.0'
      ld: warning: directory not found for option '-L/usr/local/gfortran/lib'
      ld: warning: directory not found for option '-L/usr/local/gfortran/lib/gcc/x86_64-apple-darwin15/6.1.0'
      ld: warning: directory not found for option '-L/usr/local/gfortran/lib'
      ld: library not found for -lgfortran
      clang: error: linker command failed with exit code 1 (use -v to see invocation)
      make: *** [robustbase.so] Error 1
      ERROR: compilation failed for package ‘robustbase’
      * removing ‘/Library/Frameworks/R.framework/Versions/3.4/Resources/library/robustbase’
  </code></pre>
  
  <p>I have looked around to find possible solutions and saw in <a href="https://stackoverflow.com/questions/35999874/mac-os-x-r-error-ld-warning-directory-not-found-for-option">this thread</a> that I may be needing to modify the R/makevars file. </p>
  
  <p>Is this the proper solution? If so, where do I modify this file, in my terminal? This is beyond my basic knowledge of R and code so any help is appreciated. </p>
  
  <p>Let me know if any additional information would be of help!</p>