Change `dub.sdl` when x86_omf is established

[kntroh]

See also: #60

Currently, when we try to do an x86_mscoff build with dub on Windows, We get a link error.
The following flags is required when using OPTLINK, but it must not be specified when linking COFF.

lflags `+$DWT_PACKAGE_DIR\org.eclipse.swt.win32.win32.x86\lib\` platform="x86-dmd"
lflags `/exet:nt/su:console:4.0` platform="x86-dmd"

However, as of dmd 2.093.1, I don't think it's possible to set "specify a flag for x86 but not for x86_mscoff".
It is expected that x86_omf will be added to dub's configuration in the near future, so that we can specify the flags only if it is really needed.
After x86_omf is established, we need to make changes to dub.sdl as this:

lflags `+$DWT_PACKAGE_DIR\org.eclipse.swt.win32.win32.x86\lib\` platform="x86_omf"
lflags `/exet:nt/su:console:4.0` platform="x86_omf"

[torhus]

x86_omf was added in DMD 2.094.0, maybe this can this be fixed now?

[torhus]

Actually, I think that the second line should not be there at all. I have to comment it out to avoid linker errors when using Optlink, it conflicts with /subsystem:windows:4 in my project's dub.sdl. Does /exet:nt actually do anything? I have never used it.

But the subsystem version has to be set to at least 4 when using Optlink, as it defaults to something that's too old. Otherwise you get weird issues, like Table widgets not having headers. So it probably has to be set when building examples, etc.