Enable openmp by default

See also:
https://github.com/Rdatatable/data.table/issues/6559#issuecomment-2407642567

--- configure.orig	2025-01-03 12:25:30 UTC
+++ configure
@@ -84,7 +84,7 @@ detect_openmp () {
 
 detect_openmp () {
 
-  if [ "$(uname)" = "Linux" ]; then
+  if [ "$(uname)" = "Linux" ] || [ "$(uname)" = "FreeBSD" ]; then
 
     printf "%s" "* checking if R installation supports OpenMP without any extra hints... "
     if "${R_HOME}/bin/R" CMD SHLIB test-omp.c >> config.log 2>&1; then