Change is_<os> to Sys.is<os>

[jleugeri]

JuliaLang/julia#22182 deprecates is_linux, is_windows etc in favour of Sys.islinux, Sys.iswindows etc.

Also fixes one line where a local variable was declared const.

I can't really test this atm though, because I don't get some dependencies to build properly yet.

[jleugeri]

Just saw that this is already fixed as part of some commit in #139 that hasn't been merged yet. Ignore.