WIP: use std::function if c++11 <functional> is available

[jhunold]

All except one test green.
test-tree-management-test is failing as somehow the conversion or more
exactly the display of std::function is not working as expected in
BOOST_TEST( tc1->p_test_func ); in line 118
Tested c++03 compatibility with gcc-6.2 in c++03 mode

[raffienficiaud]

Hi, thanks for the PR. I fail to understand why should we move to std::function, is there any rationale? doesn't boost::function do the job?

[jhunold]

Current clang is starting to warn about deprecated c++03/c++98 features used in the implementation boost::function. The easiest way to disable them is to simply switch to std::function if it is available. And I think that using the std:: where possible is the correct long term solution.