macOS で 'omp.h' file not found

macOS High Sierra の R (3.5.1, RStudio 1.1.383) から install.packages('caret') をすると、依存パッケージのインストール時に下のエラーが出た:

auc_.cpp:2:10: fatal error: 'omp.h' file not found
# include <omp.h>
         ^~~~~~~
1 error generated.
make: *** [auc_.o] Error 1
make: *** Waiting for unfinished jobs....
ERROR: compilation failed for package ‘ModelMetrics’
* removing ‘/usr/local/lib/R/3.5/site-library/ModelMetrics’
Warning in install.packages :
  installation of package ‘ModelMetrics’ had non-zero exit status

下のように libomp を入れると解消した:

brew install libomp