Zero initialize POD

The value were undefined, then the context could be defined with
"aborted" to any non null value for instance.
Fix Makefile (libraries after objects for linking dependencies)
Revert "size_t + printf + c++ don't go along..."

This reverts commit 887f0b8ce95ece2e3b5056a1c8a71d0e21ed1167.
Fix bit_vector (size_t is not always 32 bits...)
size_t is less likely to stay 32 bits than unsigned int

Anyway this should be replaced by a portable define, using
Boost.Integer?
Client code use type expected in jenkins_hash
 Fix bit_vector (TO FIXUP)