Fix build on NetBSD-6.99.24/amd64 with latest binutils per analysis
from Frank Kardel:

FlightCrew pulls in pthread_cancel via
/usr/include/c++/bits/gthr-default.h
/usr/include/g++/bits/gthr.h
/usr/include/g++/ext/atomicity.h
/usr/include/g++/bits/basic_string.h
/usr/include/g++/string
/usr/include/g++/random
/usr/include/g++/bits/stl_algo.h
/usr/include/g++/algorithm
/usr/pkgsrc/editors/Sigil/work/.buildlink/include/boost/utility/swap.hpp
/usr/pkgsrc/editors/Sigil/work/.buildlink/include/boost/tuple/detail/tuple_basic.hpp
/usr/pkgsrc/editors/Sigil/work/.buildlink/include/boost/tuple/tuple.hpp
/usr/pkgsrc/editors/Sigil/work/src/FlightCrew/stdafx.h
/usr/pkgsrc/editors/Sigil/work/src/FlightCrew/Result.cpp
so link against libpthread.

(wiz)