Fix AppVeyor
Fix AppVeyor

Now gcc-6.3 can’t find boost_thread and boost_system just like clang-4
:facepunch:

See https://travis-ci.org/acgetchell/CDT-plusplus/builds/234436980 vs.
https://travis-ci.org/acgetchell/CDT-plusplus/builds/237309536 and note
https://github.com/acgetchell/CDT-plusplus/commit/9a93d2becd16a4b8826f29
f95b83d36c1781d393

acgetchell/CDT-plusplus,acgetchell/CDT-plusplus,acgetchell/CDT-plusplus