build: Move ChangeLog rules in a separate file

The ChangeLog creation rules should be moved to their own file, to
make it easier to fix or change them in the future.

Makefile.am

@@ -1,4 +1,5 @@
 include $(top_srcdir)/build/autotools/Makefile.am.silent
+include $(top_srcdir)/build/autotools/Makefile.am.changelog
 
 NULL =
 
@@ -60,22 +61,3 @@ MAINTAINERCLEANFILES = 	\
 	Makefile.in 	\
 	missing 	\
 	$(NULL)
-
-#PREV_RELEASE=$(CLUTTER_MAJOR_VERSION).$$(echo "$(CLUTTER_MINOR_VERSION)-1" | bc)
-PREV_RELEASE=git-import
-
-dist-hook:
-	@if test -d "$(srcdir)/.git"; \
-	then \
-		to=""; \
-		from="$(PREV_RELEASE)"; \
-		echo Creating ChangeLog && \
-		( cd "$(top_srcdir)" && \
-		  echo '# Generated by configure.  Do no edit.'; echo; \
-		  $(top_srcdir)/missing --run perl $(top_srcdir)/build/gen-changelog.pl $$from..$$to ) > ChangeLog.tmp \
-		&& mv -f ChangeLog.tmp $(top_distdir)/ChangeLog \
-		|| ( rm -f ChangeLog.tmp ; \
-		     echo Failed to generate ChangeLog >&2 ); \
-	else \
-		echo A git checkout is required to generate a ChangeLog >&2; \
-	fi

build/autotools/Makefile.am

@@ -4,6 +4,7 @@ EXTRA_DIST = \
 	Makefile.am.silent	\
 	Makefile.am.marshal	\
 	Makefile.am.enums	\
+	Makefile.am.changelog	\
 	dolt.m4			\
 	introspection.m4	\
 	gtk-doc.m4		\

build/autotools/Makefile.am.changelog

@@ -0,0 +1,19 @@
+PREV_RELEASE=git-import
+
+changelog:
+	@if test -d "$(srcdir)/.git"; \
+	then \
+		to=""; \
+		from="$(PREV_RELEASE)"; \
+		echo Creating ChangeLog && \
+		( cd "$(top_srcdir)" && \
+		  echo '# Generated by configure.  Do no edit.'; echo; \
+		  $(top_srcdir)/missing --run perl $(top_srcdir)/build/gen-changelog.pl $$from..$$to ) > ChangeLog.tmp \
+		&& mv -f ChangeLog.tmp $(top_distdir)/ChangeLog \
+		|| ( rm -f ChangeLog.tmp ; \
+		     echo Failed to generate ChangeLog >&2 ); \
+	else \
+		echo A git checkout is required to generate a ChangeLog >&2; \
+	fi
+
+dist-hook: changelog