I discovered that freBSD systems have the dl functions in libc instead of in libdl (as it it in linux), so the libdl has not to be linked (that’s why it doesn’t exist in the std library directory).
The libdl I found in my freeBSD system is there because I enabled the linux compatibility, and so some linux features are there.
To solve the problem I had just compiled libPyROOT.so by hands without the -ldl flag.
The library compiles now.