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

* mojave install r openmp fix

* if not gcc/g++ env

* add comment

* fix

R-package/src/install.libs.R

@@ -44,6 +44,15 @@ if (!use_precompile) {
     cmake_cmd <- paste0(cmake_cmd, " -DUSE_R35=ON ")
   }
 
+  # Could NOT find OpenMP_C on Mojave workaround
+  if (Sys.info()['sysname'] == 'Darwin' && !(grepl('^gcc', Sys.getenv('CC', '')) & grepl('^g\\+\\+', Sys.getenv('CXX', '')))) {
+    cmake_cmd <- paste(cmake_cmd, ' -DOpenMP_C_FLAGS="-Xpreprocessor -fopenmp -I$(brew --prefix libomp)/include" ')
+    cmake_cmd <- paste(cmake_cmd, ' -DOpenMP_C_LIB_NAMES="omp" ')
+    cmake_cmd <- paste(cmake_cmd, ' -DOpenMP_CXX_FLAGS="-Xpreprocessor -fopenmp -I$(brew --prefix libomp)/include" ')
+    cmake_cmd <- paste(cmake_cmd, ' -DOpenMP_CXX_LIB_NAMES="omp" ')
+    cmake_cmd <- paste(cmake_cmd, ' -DOpenMP_omp_LIBRARY="$(brew --prefix libomp)/lib/libomp.dylib" ')
+  }
+
   # Check if Windows installation (for gcc vs Visual Studio)
   if (WINDOWS) {
     if (use_mingw) {