Remove the Boost_VERSION_MACRO define from C++

[vslashg]

Boost provides a header, <boost/version.hpp>, which provides a BOOST_VERSION macro with the same semantics.

This change simplifies the build process and makes this project easier to incorporate in other build systems.

Fixes # .

Changes proposed in this pull request:

@pgRouting/admins

[cvvergara]

We are only modifying documentation, as we are about to release version 3.6.0 This change has a build change, will probably go into 3.7.0 We need a description on the release_notes.rst and NEWS