Makefile: allow override of stage (revremark field for ASCIIdoctor)

Specifying STAGE=... to make now overrides the revremark value (the default
still is 'Release candidate').  E.g. 'make STAGE="public review"' will mark
the version as being for public review.

Makefile

@@ -22,10 +22,12 @@ COMMITDATE=$(shell git show -s --format=%ci | cut -d ' ' -f 1)
 bitmanip/autogenerated:
 	-mkdir $@
 
+STAGE ?= "Release candidate"
+
 bitmanip/autogenerated/revision.adoc-snippet: Makefile bitmanip/autogenerated FORCE
 	echo ":revdate: ${COMMITDATE}" > $@
 	echo ":revnumber: ${VERSION}" >> $@
-	echo ":revremark: Release candidate" >> $@
+	echo ":revremark: ${STAGE}" >> $@
 
 clean:
 	rm -f bitmanip-*.pdf