[autotools,bugfix] Fixes documentation building in tarballs.

When unpacking the tarballs, the time stamps are preserved, but the
access time of the files might be set differently. In addition during
configure Doxyfile is always created. This resulted in some files being
newer than the doxygen-tag in the tarball and triggered building the
documentation. This patch fixes the problem for me by only checking the
modification time and excluding Doxyfile.

am/doxygen

@@ -47,14 +47,14 @@ $(srcdir)/Doxyfile.in: FORCE
 
 # build doxygen when 'make doc' is called
 # what files does the doxygen-generated stuff depend on (find-syntax)
-DOXYGEN_DEPENDON = -name \*.cc -o -name \*.hh -o -name \*.png -o -name Doxyfile -o -name modules
+DOXYGEN_DEPENDON = -name \*.cc -o -name \*.hh -o -name \*.png -o -name modules
 # check dependency ourself to be robust
 $(DOXYGENTAG): FORCE Doxyfile $(DOXYGENHEADER) $(DOXYGENFOOTER)
 	set -e; \
 	DTAG=$(DOXYGENTAG); \
 	if test -f $(srcdir)/$(DOXYGENTAG); then DTAG=$(srcdir)/$(DOXYGENTAG); fi; \
 	if ! test -e "$$DTAG" || \
-	   test x"`find $(top_srcdir) \( $(DOXYGEN_DEPENDON) \) -a -cnewer $$DTAG -print | grep -v cmake/modules`" != x; \
+	   test x"`find $(top_srcdir) \( $(DOXYGEN_DEPENDON) \) -a -newer $$DTAG -print | grep -v cmake/modules`" != x; \
 	then \
 	    echo Running doxygen. This may take a while... ; \
 	    $(DOXYGEN) Doxyfile > doxygen.log <&-; \