[build] fix ChangeLog generation in release tarball

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>

Makefile.am

@@ -195,7 +195,7 @@ dist-hook: gen-ChangeLog
 gen_start_date = 2000-01-01
 .PHONY: gen-ChangeLog
 gen-ChangeLog:
-	if test -d $(abs_sourcedir)/.git; then				\
+	if test -d $(abs_srcdir)/.git; then				\
 		LC_ALL=C $(top_srcdir)/build-aux/gitlog-to-changelog	\
 			--since=$(gen_start_date) > $(distdir)/cl-t;	\
 		rm -f $(distdir)/ChangeLog;				\