Determine build directory intelligently, based on current branch

doc/Makefile

@@ -1,12 +1,28 @@
 # Minimal makefile for Sphinx documentation
 #
-
+# Possible ways to set the build directory:
+# (1) Set BUILDDIR from the command line
+# (2) Let BUILDDIR be determined as $(BUILDREPO)/cism-in-cesm/$(VERSION)
+#     (2a) Set both BUILDREPO and VERSION directly on the command line
+#     (2b) Set BUILDREPO on the command line, let VERSION be auto-determined,
+#          based on the current git branch name.
+#
 # You can set these variables from the command line.
 SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
 SPHINXPROJ    = cismdoc
 SOURCEDIR     = source
-BUILDDIR      = build
+
+ifndef BUILDDIR
+# If version isn't set on the command-line, then we set it here, by
+# getting the current git branch name.
+VERSION ?= $(shell git symbolic-ref --short -q HEAD)
+ifeq ($(VERSION),)
+$(error Cannot determine version based on git branch; set VERSION on the command line)
+endif
+BUILDDIR = $(BUILDREPO)/cism-in-cesm/$(VERSION)
+$(info Using build directory: $(BUILDDIR))
+endif
 
 # Put it first so that "make" without argument is like "make help".
 help:
@@ -17,4 +33,4 @@ help:
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)