In some projects, includes are required to be relative to the top of the hierarchy. This allows you to have a single include directory, typically with an absolute path, and simply include headers by their full path relative that location.
g++ -I%FIR_ECOSYSTEM_ROOT% foo.cc
In which foo.cc would include files in a manner like:
By retaining the path information in the include directive you gain location independence and the ability to distinguish between several headers with the same filename.