$NetBSD$

Handle OpenMP on Darwin, where native clang does not support -fopenmp.
See https://mac.r-project.org/openmp/.

--- src/Makefile.orig	2019-05-19 02:52:11.000000000 +0000
+++ src/Makefile
@@ -1,5 +1,5 @@
 # Define required macros here
-CFLAGS = -Wall -std=c++11 -O3 -fopenmp
+CFLAGS = -Wall -std=c++11 -O3 ${OPENMP_CFLAGS}
 CC = g++
 INCLUDES = -Igrf-main -Igrf-intersperse -Igrf-mite-cluster