fix for targetextension config option

[p0358]

With Premake you can specify the option to change target file's extension like this: targetextension ".aes"

This would cause the check I fixed to fail, as the extension isn't .dll. Not sure if my fix for this is perfect, but I don't think MSVC can build non-Windows libraries, so it should be fine?

This fails the gcc test though, but honestly I don't have any better ideas for now. But for now better to partially fix than not at all, unless you have any better ideas...

[Jarod42]

Unclear what you expect to happens with targetextension ".aes"...
I agree that current test for extra files is not ideal for now...

[p0358]

It should simply override the output file's extension from the default exe and dll, it's supported by core, and in my case I'm compiling a replacement DLL where someone originally for some reason decided to just rename the extension from .dll to .aes (it doesn't make much sense, but there is such option and Windows has no issues with that either)

[Jarod42]

Should be fixed by 3077d47

using os.target() == "windows" for dll/lib stuff.