Yes, Win32threads.
About ffms2 issue linking EXE file I'm using
this patch (line #1348 + #1349)
About ffms2 integration in x264 this works fine
--extra-ldflags="$(PKG_CONFIG_PATH=/usr/i686-w64-mingw32/lib/pkgconfig pkg-config --libs --static ffms2)". Just tested with 8.3.1 x86 from that gcc-mcf link.