Currently we use "<dir><api.location.file>" as \file argument, and as
base for the CPP guard. This is not nice when <dir> is absolute, in
which case it is expected that the user will use api.location.include
to get something nicer. If defined, use that name instead.
* data/location.cc (b4_location_path): New.
Use it.
* tests/c++.at (Shared locations): Check the guard and Doxygen doc.