rPackages.ModelMetrics: fix darwin build

This fixes the error:

	auc_.cpp:2:10: fatal error: 'omp.h' file not found
	#include <omp.h>

pkgs/development/r-modules/default.nix

@@ -277,6 +277,7 @@ let
     KFKSDS = [ pkgs.gsl_1 ];
     kza = [ pkgs.fftw.dev ];
     magick = [ pkgs.imagemagick.dev ];
+    ModelMetrics = lib.optional stdenv.isDarwin pkgs.llvmPackages.openmp;
     mvabund = [ pkgs.gsl_1 ];
     mwaved = [ pkgs.fftw.dev ];
     ncdf4 = [ pkgs.netcdf ];
@@ -735,6 +736,11 @@ let
         + lib.optionalString stdenv.isDarwin " -fopenmp";
     });
 
+    ModelMetrics = old.ModelMetrics.overrideDerivation (attrs: {
+      NIX_CFLAGS_COMPILE = attrs.NIX_CFLAGS_COMPILE
+        + lib.optionalString stdenv.isDarwin " -fopenmp";
+    });
+
     rpf = old.rpf.overrideDerivation (attrs: {
       patchPhase = "patchShebangs configure";
     });