Fix build failure when update-doxygen.css is unavailable

[[Imported from SVN: r5615]]

am/doxygen

@@ -64,12 +64,15 @@ DOXYGEN_DEPENDON = -name \*.cc -o -name \*.hh -o -name \*.png -o -name Doxyfile
 # check dependency ourself to be robust
 doxygen-tag: FORCE Doxyfile $(DOXYGENHEADER) $(DOXYGENFOOTER)
 	set -e; \
-	  if test ! -e doxygen-tag || test x"`find $(top_srcdir) \( $(DOXYGEN_DEPENDON) \) -a -cnewer doxygen-tag -print`" != x ; then \
+	if test ! -e doxygen-tag || test x"`find $(top_srcdir) \( $(DOXYGEN_DEPENDON) \) -a -cnewer doxygen-tag -print`" != x ; then \
 	    echo Running doxygen. This may take a while... ; \
 	    $(DOXYGEN) $(srcdir)/Doxyfile > doxygen.log 2>&1 <&- ; \
 	    touch doxygen-tag ; \
 		if test "$$DOXYQUIET" != "1" && test -f doxyerr.log; then cat doxyerr.log; fi; \
 	    test -x "$(DUNEWEBDOXY)/update-doxygen.css" && $(DUNEWEBDOXY)/update-doxygen.css $(abs_builddir)/html; \
+	    # Make sure to always return a true exit code (even when the	\
+            # previous test failed)						\
+	    true;								\
 	fi
 
 doxygen-dist-install: doxygen-tag