build: fix automake error due to missing ChangeLog

Apparently, Automake does not accept the '$(srcdir)/ChangeLog'
target in Makefile.am as a declaration that ChangeLog is
automatically generated (and thus does not need to exist at
automake time).  One has to use a *literal* 'ChangeLog' target.

Problem introduced in commit v2.68-118-g6ed5195 of 2012-01-17,
"maint: generate ChangeLog from git log".

* Makefile.ma ($(srcdir)/ChangeLog): Renamed ...
(ChangeLog): ... to this.

Makefile.am

@@ -64,7 +64,7 @@ gen-ChangeLog:
 # bootstrap.  Creates a dummy ChangeLog (unless one is already present,
 # as might be the case for users of vc-dwim) that will be overridden
 # with a proper one at distribution time.
-$(srcdir)/ChangeLog:
+ChangeLog:
 	@echo dummy > $@
 
 # Version string management.  There are two files to be aware of: