pg_upgrade / Checking for presence of required libraries

Hello,
we are trying pg_upgrade on an old test instance (9.1 -> 9.3)

the check fails for a library that we don't mange to locate in the old system.
(we suspect it to come from a very old try to implement a custom FTS parser).

So my question: can we find out where the pg_ugrade checker did find this reference?

($libdir/hello.so does not exist in the 9.1 instance)


  Performing Consistency Checks
  -----------------------------
  ...
  Checking for presence of required libraries                 fatal

  Could not load library "$libdir/hello.so"
  ERROR:  could not access file "$libdir/hello.so": No such file or directory

many thanks,

Marc Mamin