Enable OpenMP on macOS

Author: bkpoon

.azure-pipelines/xfel/unix-conda-build.yml

@@ -70,6 +70,9 @@ steps:
     conda create -y -n $(PYTHON_VERSION) --file $(Pipeline.Workspace)/modules/dials/.conda-envs/$(OS).txt python=$(PYTHON_FULL_VERSION)
     conda install -y -c conda-forge --no-deps -n $(PYTHON_VERSION) junit-xml
     conda install -y -c conda-forge cmake pandas distro -n $(PYTHON_VERSION)
+    if [[ ! -z "$MACOSX_DEPLOYMENT_TARGET" ]]; then
+      conda install -y -c conda-forge llvm-openmp
+    fi
   displayName: Create conda environment
 
 # build