Detect including the same idp file via different paths

[Broes De Cat]

Currently, the parser detects detection of including the same file twice only when their include paths are the same (when expanded according to the position of the include path). It should be improved by detecting that 2 paths in fact refer to the same file.

The only solution found is the use of Boost::FileSystem::equivalent(file, file). This library is OS-dependent, so the user should install it themselves, in which case we can implement a configure check for the library.