Don't clean HTML manual if we didn't build it

Clean the HTML manual upon 'maintainer-clean' rather than 'clean' in
case it was not built by Make but rather part of the distribution.
This is fine even then, as configure will properly require what is
needed to build it again if it is missing.

doc/Makefile.am

@@ -58,7 +58,11 @@ hacking.html: $(srcdir)/geany.css $(top_srcdir)/HACKING
 
 all-local: geany.html hacking.html
 
-clean-local: clean-html-local
+# clean on 'maintainer-clean' rather than 'clean' in case it was not
+# built by Make but rather part of the distribution.  This is fine even
+# then, as configure will properly require what is needed to build it
+# again if it is missing.
+maintainer-clean-local: clean-html-local
 clean-html-local:
 	-rm -f hacking.html
 	-rm -f geany.html