Add missing generated files to generator_built and EXTRA_DIST.

Use the generator_built variable to list any files which are built by
the generator.  This ensures they get automatically rebuilt.

Three files were missing from EXTRA_DIST, so they were not included in
the tarball, meaning you would have needed OCaml to build from the
tarball.  You would have seen this when building:

  cd .. && /builddir/build/BUILD/libguestfs-1.31.9/generator/generator
  written gobject/docs/guestfs-title.sgml
  written customize/customize-synopsis.pod
  written customize/customize-options.pod
  generated 476921 lines of code

customize/Makefile.am

@@ -1,5 +1,5 @@
 # virt-customize
-# Copyright (C) 2014 Red Hat Inc.
+# Copyright (C) 2014-2015 Red Hat Inc.
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -18,6 +18,7 @@
 include $(top_srcdir)/subdir-rules.mk
 
 EXTRA_DIST = \
+	$(generator_built) \
 	$(SOURCES_MLI) $(SOURCES_ML) $(SOURCES_C) \
 	test-virt-customize.sh \
 	virt-customize.pod

gobject/docs/Makefile.am

@@ -1,5 +1,5 @@
 # libguestfs
-# Copyright (C) 2012 Red Hat Inc.
+# Copyright (C) 2012-2015 Red Hat Inc.
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -17,6 +17,8 @@
 
 include $(top_srcdir)/subdir-rules.mk
 
+generator_built = guestfs-title.sgml
+
 # The name of the module, e.g. 'glib'.
 DOC_MODULE=guestfs
 
@@ -94,7 +96,7 @@ include $(srcdir)/gtk-doc.make
 
 # Other files to distribute
 # e.g. EXTRA_DIST += version.xml.in
-EXTRA_DIST +=
+EXTRA_DIST += $(generator_built)
 
 # Files not to distribute
 # for --rebuild-types in $(SCAN_OPTIONS), e.g. $(DOC_MODULE).types