osx makefile backporting of commit be34510a9a17b3be83e817d6e58c9023a48e75bc
Fixed the ubuntu building doc

Tested on pristine Ubuntu 12.04 LTS and 13.10
disable safemode & add checkpoint
bugfix: weak checking for existing users (the transaction may exist in index only
but the chain it belongs was invalidated) caused fork between users who erased their
block database from yesterday and those who didn't. improve the checking (test
if tx is actually in main chain). add another checkpoint. enforce.
Merge pull request #49 from toyg/osx_makefile_backport

osx makefile backport of be34510
Merge pull request #51 from lra/better_ubuntu_doc

Fixed the ubuntu building doc