build: add dataclean

It turns out to be inconvenient to delete the downloaded datafiles with
distclean, so I propose a new target which does that instead.

The closest conventional target is 'maintainer-clean'; the difference
here is that having the original source tarball is not enough to
reconstruct these files.

Makefile.local

@@ -262,6 +262,10 @@ clean:
 distclean: clean
 	rm -rf $(DISTCLEAN)
 
+.PHONY: dataclean
+dataclean: distclean
+	rm -rf $(DATACLEAN)
+
 notmuch_client_srcs =		\
 	command-line-arguments.c\
 	debugger.c		\

performance-test/Makefile.local

@@ -41,3 +41,4 @@ download-corpus:
 
 CLEAN := $(CLEAN) $(dir)/tmp.* $(dir)/log.*
 DISTCLEAN := $(DISTCLEAN) $(dir)/corpus $(dir)/notmuch.cache.*
+DATACLEAN := $(DATACLEAN) $(TXZFILE)

test/test-databases/Makefile.local

@@ -11,4 +11,4 @@ test_databases := $(dir)/database-v1.tar.xz
 
 download-test-databases: ${test_databases}
 
-DISTCLEAN := $(DISTCLEAN) ${test_databases}
+DATACLEAN := $(DATACLEAN) ${test_databases}