title: cannot install package OpenMx in R
body: <p>I'm running R version 3.5 on a mac.
  When I try to install OpenMx by</p>
  
  <pre><code>install.packages("OpenMx")
  </code></pre>
  
  <p>it eventually fails after a bunch of warning messages.</p>
  
  <p>At first the warning message was</p>
  
  <pre><code>xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
  </code></pre>
  
  <p>and found out this could be handled by running</p>
  
  <pre><code>xcode-select --install
  </code></pre>
  
  <p>on Terminal.</p>
  
  <p>When I tried installing OpenMx after installing xcode,</p>
  
  <p>the last several lines of messages are:</p>
  
  <pre><code>optimize.c:35:15: warning: unused function 'f_bound' [-Wunused-function]
  static double f_bound(int n, const double *x, void *data_)
                ^
  optimize.c:51:15: warning: unused function 'f_noderiv' [-Wunused-function]
  static double f_noderiv(int n, const double *x, void *data_)
                ^
  optimize.c:57:15: warning: unused function 'f_direct' [-Wunused-function]
  static double f_direct(int n, const double *x, int *undefined, void *data_)
                ^
  optimize.c:79:21: warning: unused function 'initial_step' [-Wunused-function]
  static nlopt_result initial_step(nlopt_opt opt, const double *x, double *step)
                      ^
  optimize.c:101:12: warning: unused function 'finite_domain' [-Wunused-function]
  static int finite_domain(unsigned n, const double *lb, const double *ub)
             ^
  5 warnings generated.
  clang -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG  -I"/Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include" -I"/Library/Frameworks/R.framework/Versions/3.5/Resources/library/RcppEigen/include" -I"/Library/Frameworks/R.framework/Versions/3.5/Resources/library/StanHeaders/include" -I"/Library/Frameworks/R.framework/Versions/3.5/Resources/library/BH/include" -I"/Library/Frameworks/R.framework/Versions/3.5/Resources/library/rpf/include" -I/usr/local/include   -fPIC  -Wall -g -O2  -c options.c -o options.o
  gfortran   -fPIC  -g -O2  -c sadmvn.f -o sadmvn.o
  make: gfortran: No such file or directory
  make: *** [sadmvn.o] Error 1
  ERROR: compilation failed for package ‘OpenMx’
  * removing ‘/Library/Frameworks/R.framework/Versions/3.5/Resources/library/OpenMx’
  Warning in install.packages :
    installation of package ‘OpenMx’ had non-zero exit status
  
  The downloaded source packages are in
  ‘/private/var/folders/f8/y98w5w9n3yz3sq2pthhw35xm0000gn/T/RtmpJtQ01E/downloaded_packages’
  </code></pre>
  
  <p>Is there a way to fix this?</p>