doc/Makefile.am: respect DESTDIR variable

The old behavior didn't allow a additonal setting of the DESTDIR
variable. This is a serious problem while building within a chroot for
example. Also the implementation of the stamp file was triggering
multiple builds in some cirumstances. This handling is reverted with the
remove of the old stamp file handling.

doc/Makefile.am

@@ -10,25 +10,32 @@
 
 if HAVE_DOXYGEN
 
-dist_pkgdata_DATA = doxyfile.stamp
-
 CLEANFILES = \
-  doxyfile.stamp \
   doxygen_sqlite3.db
 
-doxyfile.stamp:
-	$(DOXYGEN) Doxyfile
-	echo Timestamp > doxyfile.stamp
 
-all-local: doxyfile.stamp
+# This target has no check for automated install data! It's simply assumed that
+# that the 'all' target is running before, in short: the user has to run first
+# 'make' before running 'make install'!
+install-data-am:
+	@if [ ! -d $(srcdir)/html ]; then \
+		echo ;\
+		echo "     No install data in '$(srcdir)/html'found! Please run 'make all' first." ;\
+		echo ;\
+		exit 1 ;\
+	fi
+	$(MKDIR_P) $(DESTDIR)$(htmldir)/html || exit 1
+	cp -a $(srcdir)/html $(DESTDIR)$(htmldir)
+
+uninstall-am:
+	-rm -rf $(DESTDIR)$(htmldir)/html
+
+all:
+	$(DOXYGEN) Doxyfile
 
 clean-local:
 	rm -rf $(srcdir)/html
 
-install-data-am: all-local
-	$(MKDIR_P) $(htmldir)/html || exit 1
-	cp -a $(srcdir)/html $(htmldir)
-
-.PHONY: doxyfile.stamp
+distclean-local: clean-local
 
 endif # HAVE_DOXYGEN