The mingw-w64 version on the CI platform is older and
is missing a sought copy of winres.h.
This attempts to work around that by having the Makefile
create a temporary copy of winres.h in the win/win32 directory
which that windres.exe is already search in. The file is
then immediately removed after windres uses it.
The contents of the temporary winres.h match the contents
of that file that is distributed with the more up-to-date msys2
distribution of mingw-w64.
It won't be known if this workaround solves all the CI issues
with the mingw build until after it is committed and observed.
c03f24ceb7