Adapting Makefile to assure appcache build

It's important that the version number in appcached and index/mobile files are the same. By making appcache build depend on index/mobile, this is assured.

@oterral I stumbled upon this during some tests. You might rebuild index.html, but not appcache...and your appcache is dead.
it's already the case in src/geoadmin.mako.appcache traget