Define METPLUS_ROOT in the Makefile for the docs.

doc/Makefile

@@ -1,10 +1,12 @@
 # Makefile for Sphinx documentation
 
-SPHINXOPTS    = -a -n -W
+#SPHINXOPTS    = -a -n -W
+SPHINXOPTS    = -a -n
 SPHINXBUILD   = sphinx-build
 SOURCEDIR     = .
 BUILDDIR      = build
 LINKCHECKDIR  = $(BUILDDIR)/linkcheck
+METPLUSROOT   = /scratch1/NCEPDEV/nems/role.epic/spack-stack/spack-stack-1.6.0/envs/upp-addon-env/install/intel/2021.5.0/metplus-5.1.0-eznr7ze
 
 .PHONY: help Makefile linkcheck
 
@@ -14,8 +16,8 @@ help:
 
 doc:
 	make clean
-	$(MAKE) linkcheck
-	$(MAKE) html
+	export METPLUS_ROOT=$(METPLUSROOT) ; $(MAKE) linkcheck
+	export METPLUS_ROOT=$(METPLUSROOT) ; $(MAKE) html
 
 linkcheck:
 	$(SPHINXBUILD) -b linkcheck $(SPHINXOPTS) $(SOURCEDIR) $(LINKCHECKDIR)
@@ -24,4 +26,4 @@ linkcheck:
 # "make mode" option. $(O) is meant as a shortcut for $(SPHINXOPTS).
 
 %: Makefile
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) -w $(BUILDDIR)/warnings.log
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) -w $(BUILDDIR)/warnings.log