Now i'm on the multi-core computer and the problem remain as before at this point i think is present some bug on the:
$ make -version
GNU Make 3.81
since my only multi-core pc is an old amd dual-core i don't known if that problem is present only on dual-core pc or on every multi-core pc, the difference is important for any attempt to report a similar tricky bug ...
On the multi-core Pc use only:
$ make test-forum1
the file with extension .o isn't generated (patience) but at least the executable is built.
And if the worst is to come on the single core set the program using two threads although this would not allow me to do any tests to the level of performance improvement obtained after the addition of openmp.
It was important for me the help for the makefile because I would like to make changes to a program built using makefiles.
Thanks for the help.