Adds the ability to override the doc build output directory from the command line. (#36604)

* Adds the ability to override the doc build output from the command line.

* For safety, removed straight rm of BUILDDIR and removed subdirectories instead.

* Added check to see if BUILDDIR was defined to main makefile

docs/docsite/Makefile

@@ -11,6 +11,11 @@ else
 CPUS ?= $(shell nproc)
 endif
 
+# Sets the build output directory if it's not already specified
+ifndef BUILDDIR
+	BUILDDIR = _build
+endif
+
 MODULE_ARGS=
 ifdef MODULES
 	MODULE_ARGS = -l $(MODULES)
@@ -44,6 +49,9 @@ webdocs: docs
 
 #TODO: leaving htmlout removal for those having older versions, should eventually be removed also
 clean:
+	@echo "Cleaning $(BUILDDIR)"
+	-rm -rf $(BUILDDIR)/doctrees
+	-rm -rf $(BUILDDIR)/html
 	-rm -rf htmlout
 	-rm -rf module_docs
 	-rm -rf _build
@@ -103,5 +111,5 @@ epub:
 	(CPUS=$(CPUS) make -f Makefile.sphinx epub)
 
 htmlsingle: assertrst
-	sphinx-build -j $(CPUS) -b html -d _build/doctrees ./rst _build/html rst/$(rst)
-	@echo "Output is in _build/html/$(rst:.rst=.html)"
+	sphinx-build -j $(CPUS) -b html -d $(BUILDDIR)/doctrees ./rst $(BUILDDIR)/html rst/$(rst)
+	@echo "Output is in $(BUILDDIR)/$(rst:.rst=.html)"

docs/docsite/Makefile.sphinx

@@ -6,7 +6,11 @@ SPHINXOPTS    = -j $(CPUS) -n -w rst_warnings
 SPHINXBUILD   = sphinx-build
 SPHINXPROJ    = sdfsdf
 SOURCEDIR     = rst
+
+# Sets the build output directory if it's not specified on the command line
+ifndef BUILDDIR
    BUILDDIR      = _build
+endif
 
 # Put it first so that "make" without argument is like "make help".
 help: