Add support for FreeBSD

[smortex]

In order to get the current thread id on FreeBSD, we can use the non-portable pthread_getthreadid_np() function.

Test suite pass:

romain@zappy ~/Projects/vernier % bundle exec rake                                                                                             
mkdir -p tmp/amd64-freebsd14/vernier/3.2.3
cd tmp/amd64-freebsd14/vernier/3.2.3
/usr/local/bin/ruby32 -I. ../../../../ext/vernier/extconf.rb
checking for ruby/thread.h... yes
checking for rb_internal_thread_event_data_t.thread in ruby/thread.h... no
checking for rb_profile_thread_frames() in ruby/debug.h... no
checking for pthread_setname_np()... yes
creating Makefile
cd -
cd tmp/amd64-freebsd14/vernier/3.2.3
/usr/local/bin/gmake
compiling ../../../../ext/vernier/vernier.cc
linking shared-object vernier/vernier.so
cd -
mkdir -p tmp/amd64-freebsd14/stage/lib/vernier
/usr/local/bin/gmake install sitearchdir=../../../../lib/vernier sitelibdir=../../../../lib/vernier target_prefix=
/usr/bin/install -c -m 0755 vernier.so ../../../../lib/vernier
cp tmp/amd64-freebsd14/vernier/3.2.3/vernier.so tmp/amd64-freebsd14/stage/lib/vernier/vernier.so
Run options: --seed 35046

# Running:

............................................S.......

Finished in 4.508479s, 11.5338 runs/s, 2226.6935 assertions/s.

52 runs, 10039 assertions, 0 failures, 0 errors, 1 skips

You have skipped tests. Run with --verbose for details.
romain@zappy ~/Projects/vernier %