allow compilation of documentation without changelog files

doc/Makefile

@@ -5,10 +5,9 @@
 aspect-files      = $(shell echo ../include/aspect/*.h \
                             ../include/aspect/*/*.h     \
                             ../include/aspect/*/*/*.h   \
-                            modules/*.h \
-                            modules/changes/*)
+                            modules/*.h)
 aspect-manual-pix = $(shell find manual/cookbooks/ -name \*.png 2>/dev/null)
-
+change-files = $(shell ls modules/changes)
 
 aspect-prms = $(shell find manual/cookbooks/ -name \*.prm 2>/dev/null)
 aspect-prms-out = $(aspect-prms:=.out)
@@ -32,14 +31,14 @@ all: aspect.tag manual.pdf
 	@for i in `seq 1 10`; do sed -i.bak 's/{\([^!]*\)!\([^!]*\)!\([^!]*\)!\([^}]*\)}/{\1!\2!\3\/\4}/' $@; rm $@.bak; done
 
 # Generate changes.h from the bits in modules/changes/
-modules/changes.h: modules/changes/* modules/create_changes.sh modules/current_changes_*
+modules/changes.h: $(change-files) modules/create_changes.sh modules/current_changes_*
 	@cd modules && bash create_changes.sh
 
 # build the documentation. see if we can download the latest
 # deal.II deal.tag file generated by doxygen and if so
 # use it to cross-reference the deal.II documentation from the
 # aspect documentation
-aspect.tag: $(aspect-files) modules/changes.h options.dox Makefile
+aspect.tag: $(aspect-files) $(change-files) modules/changes.h options.dox Makefile
 	@if test ! -x "`which doxygen`" ; then \
 	  echo "-----------------------------------------" ; \
 	  echo "Can't find 'doxygen'." ; \