Segfault in libperl when doing perl_xlat(). Fix by kvainkainen. Closes #436

In rlm_perl.c Perl interpreter cloning is controlled by #ifdef USE_ITHREADS.
However, for some unknown reason in perl_xlat() #ifndef WITH_ITHREADS is
used instead. At least my Perl does not have WITH_ITHREADS compile option,
meaning all perl_xlat() calls are handled by same Perl instance.

If you run Freeradius in multithread mode, this causes segfault in libperl
when two threads do perl_xlat() simultaneously.