Fix "make dist" work out of the box again

ChangeLog is listed as EXTRA_DIST but there's no rule to create it
so dist target is broken except when invoked with Makefile.maint. Which
nobody finds because its such a strange thing to have.

Move back ChangeLog generation into main Makefile.am but do not
require git to create it. Instead have a rule to create an empty
file to appease EXTRA_DIST no matter what, and only create the
real thing if we're in a git checkout and git command is present.

Makefile.am

@@ -302,6 +302,12 @@ BUILT_SOURCES += db.h
 endif
 DISTCLEANFILES = db.h
 
+ChangeLog:
+	touch ChangeLog
+
+dist-hook:
+	if [ -d .git ] && [ -x $(command -v git) ]; then git log --no-merges > $(distdir)/ChangeLog; fi
+
 clean-local:
 	rm -rf doc/hacking/html
 	rm -rf include

Makefile.maint

@@ -1,9 +1,4 @@
 include Makefile
 
-snapshot: ChangeLog
+snapshot:
 	@$(MAKE) dist VERSION=$(VERSION)-git`git rev-list --count HEAD`
-
-release: ChangeLog dist
-
-ChangeLog: .git
-	@git log --no-merges > ChangeLog