Rework ChangeLog generation rule

Generation target is now "cl" and also works when build dir is not
below source dir.  ChangeLog is pushed to repo.

HOWTO_RELEASE

@@ -1,9 +1,11 @@
-1. Set version in configure.ac
-2. Run make distcheck, fix if needed
-3. Push to repository, confirm bots are all happy
-4. Add annotated tag: git tag -sa librttopo-<version>
-5. Push annotated tag to repository: git push <remote> librttopo-<version>
-6. Create release artifacts: make dist
-7. Upload release artifacts to upload.osgeo.org
-8. Update websites (to be defined)
-9. Announce on discuss@lists.osgeo.org
+- Set version in configure.ac
+- Run make distcheck, fix if needed
+- Commit all of the above
+- Regenerate ChangeLog (make cl) and commit again
+- Commit and push to repository, confirm bots are all happy
+- Add annotated tag: git tag -sa librttopo-<version>
+- Push annotated tag to repository: git push <remote> librttopo-<version>
+- Create release artifacts: make dist
+- Upload release artifacts to upload.osgeo.org
+- Update websites (to be defined)
+- Announce on discuss@lists.osgeo.org

Makefile.am

@@ -17,8 +17,6 @@ AUTOMAKE_OPTIONS = dist-zip foreign
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = rttopo.pc
 
-.PHONY: ChangeLog
-
-ChangeLog:
-	git2cl > $(srcdir)/$@
+cl:
+	cd $(srcdir) && git2cl > ChangeLog