[R-package] mojave install r openmp fix (#2164)
[R-package] mojave install r openmp fix (#2164)

* mojave install r openmp fix

* if not gcc/g++ env

* add comment

* fix

henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM