Add sehandler.hxx with the magic __SEHandler class for MinGW SEH
Add sehandler.hxx with the magic __SEHandler class for MinGW SEH

From the crosswin32-dtrans-mingw.diff patch in the build repo. Also in
janneke's gub, see
https://github.com/janneke/gub/commit/8ba48182c026432f72fc40752017a31b4eab60f8,
but whether this was originally written by janneke, or jcorrius, or
somebody else, no idea, sigh.

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core