I could not find for sure what to use on FreeBSD instead, so let's just
not get this information there. It is quite useful information to know
where thread traces were asked from, but it is more important to make
sure the program can be compiled everywhere. Also we can just check
which thread has gimp_stack_trace*() calls. Thus it can be seen as
redundant information in any case.
SYS_gettid is apparently defined as a macro, so let's simply check for
it being defined.
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