You can use this C++11 implementation
https://github.com/Cwc-Test/mingw-std-threads instead that Komisar's patch.
Copy all mingw.*.h files to /mingw32/include/c++/**/
** - GCC version
Then from ffms2 root folder:
sed -i 's/#include <mutex>/#include "mingw.mutex.h"/g' src/core/ffms.cpp
sed -i 's/#include <thread>/#include "mingw.thread.h"/g' src/core/videosource.cpp