science/cantera: Fix build on FreeBSD 12

c++ -o build/src/base/global.os -c -isystem include/cantera/ext -isystem /usr/local/include -isystem /usr/local/include/eigen3 -std=c++17 -O2 -pipe -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -isystem /usr/local/include -pthread -O3 -Wall -include-pch src/pch/system.h.gch -fPIC -DNDEBUG -DGIT_COMMIT=\"unknown\" -Iinclude -Ibuild/src src/base/global.cpp
In file included from src/base/global.cpp:11:
In file included from /usr/local/include/boost/stacktrace.hpp:15:
In file included from /usr/local/include/boost/stacktrace/frame.hpp:38:
In file included from /usr/local/include/boost/stacktrace/safe_dump_to.hpp:217:
/usr/local/include/boost/stacktrace/detail/collect_unwind.ipp:101:7: error: no member named '_Unwind_Backtrace' in the global namespace

::_Unwind_Backtrace(&boost::stacktrace::detail::unwind_callback, &state);
~~^

1 error generated.