Turn macOS support for OpenMP off

Author: eddelbuettel

configure

@@ -107,6 +107,8 @@ detect_openmp () {
   fi # uname=Linux
 
   if [ "$(uname)" = "Darwin" ]; then
+    export R_OPENMP_ENABLED=0
+    return
 
     # https://mac.r-project.org/openmp
     printf "%s" "* checking if R installation supports OpenMP with \"-Xclang -fopenmp\" ... "