Allow installation on non-compatible OSes

[simonbyrne]

Given that Pkg3 doesn't support OS-specific dependencies (JuliaLang/Pkg.jl#165), there needs to be a way to install this on incompatible OSes. The main problem is deps/build.jl (which is called at installation time) and attempts to call WinRPM.update(): could this be removed?

[simonbyrne]

Alternatively we could put it behind an iswindows() check? (though from what I understand it also works on rpm-based linux distros?)

[tkelman]

It should work on non-rpm distros as well, theoretically. Has been used in the past to download the same windows cross-compiled libraries from Linux or Mac for testing purposes in wine or similar.

[simonbyrne]

Do we need to run update on installation?

[tkelman]

Can't remember what the need/justification for that is - but given the custom applications this has only ever been used for on non-Windows, making that particular step Windows-only by default should be fine.

[NHDaly]

Thanks for taking the initiative to fix this, Simon! :)