I do not really understand why this is necessary. The problem is that /nonfatal conflicts with our msi creation as there is no such equivalent in the wix toolset language.

My workaround for this was to always create / package both files with the one that is unused just an empty file. In configure.ac we have this:

if test -f "/usr/lib/gcc/i686-w64-mingw32/10-win32/libgcc_s_dw2-1.dll"; then
    AC_MSG_NOTICE([Using dw2 exceptions.])
    GPG4WIN_RUNTIME_LIBRARY(libgcc_s_dw2-1)
    $CP /dev/null src/libgcc_s_sjlj-1.dll-x
else
    AC_MSG_NOTICE([Using sjlj exceptions.])
    GPG4WIN_RUNTIME_LIBRARY(libgcc_s_sjlj-1)
    $CP /dev/null src/libgcc_s_dw2-1.dll-x
fi

Did this not work for you? I would like to revert this commit.