This line breaks our Windows build on MSVC.  We'll need to do something else here.  Details:

codec/encoder/core/src/slice_multi_threading.cpp(356) : error C2065: 'unused' :
undeclared identifier
codec/encoder/core/src/slice_multi_threading.cpp(356) : error C2146: syntax erro
r : missing ';' before identifier 'WELS_THREAD_ERROR_CODE'
codec/encoder/core/src/slice_multi_threading.cpp(356) : error C3861: '__attribut
e__': identifier not found
codec/encoder/core/src/slice_multi_threading.cpp(356) : error C2146: syntax erro
r : missing ';' before identifier 'err'
codec/encoder/core/src/slice_multi_threading.cpp(356) : error C2275: 'WELS_THREA
D_ERROR_CODE' : illegal use of this type as an expression
        c:\Users\Administrator\github\ylatuya\openh264\codec\common\WelsThreadLi
b.h(89) : see declaration of 'WELS_THREAD_ERROR_CODE'
codec/encoder/core/src/slice_multi_threading.cpp(356) : error C2065: 'err' : und
eclared identifier
codec/encoder/core/src/slice_multi_threading.cpp(366) : error C2065: 'err' : und
eclared identifier
codec/encoder/core/src/slice_multi_threading.cpp(369) : error C2065: 'err' : und
eclared identifier
codec/encoder/core/src/slice_multi_threading.cpp(373) : error C2065: 'err' : und
eclared identifier
codec/encoder/core/src/slice_multi_threading.cpp(376) : error C2065: 'err' : und
eclared identifier
codec/encoder/core/src/slice_multi_threading.cpp(379) : error C2065: 'err' : und
eclared identifier
codec/encoder/core/src/slice_multi_threading.cpp(382) : error C2065: 'err' : und
eclared identifier
codec/encoder/core/src/slice_multi_threading.cpp(390) : error C2065: 'err' : und
eclared identifier
codec/encoder/core/src/slice_multi_threading.cpp(730) : warning C4018: '>=' : si
gned/unsigned mismatch
codec/encoder/core/src/slice_multi_threading.cpp(1072) : warning C4018: '>=' : s
igned/unsigned mismatch
make: *** [codec/encoder/core/src/slice_multi_threading.o] Error 2